How to Fix P20EE Code: From Diagnosis to Repair

Common in diesel engines, the P20EE Diagnostic Trouble Code (DTC) indicates a fault with the nitrogen oxide (NOx) catalyst efficiency or the Selective Catalytic Reduction (SCR) system. 

This comprehensive guide to diagnosing and resolving the P20EE engine code issue offers auto technicians valuable information on common causes, step-by-step diagnostic procedures, and the protocols required to complete the repair. 

Use the following instructions and troubleshooting tips to identify and fix the root cause of the P20EE code. 

Key Takeaways

• P20EE is a calculated verdict on SCR system efficiency, so the code names the catalyst without proving the catalyst is what failed.
• DEF quality, NOx sensor drift, and outdated calibration cause far more of these than a worn catalyst, and a stored P207F or P2201 tells you which one to chase.
• The power loss customers complain about comes from the inducement countdown rather than the fault itself, and clearing the code does not send that counter back to the top.

What Does P20EE Mean?

The DTC P20EE signals an issue with the diesel engine’s catalyst efficiency, which is required for it to comply with emissions control regulations. In a nutshell, the SCR system monitors the NOx concentration in the exhaust stream. 

When NOx levels exceed allowable limits, the SCR system strategically injects diesel exhaust fluid (DEF) into the hot exhaust stream to reduce NOx by converting it into nitrogen, carbon dioxide, and water. 

When the SCR system detects a discrepancy in NOx readings along the exhaust stream, it signals a P20EE DTC, specifically “SCR NOx Catalyst Efficiency Below Threshold,” often followed by an engine bank designation. 

Driving a short distance with this code will not significantly damage the vehicle. However, failure to address the issue can damage the emissions system and potentially require a catalytic converter replacement. 

Vehicles Commonly Affected by P20EE

As stated previously, the P20EE engine code can only be found on diesel-powered vehicles. Gas-powered vehicles may contain similar emissions systems but do not reference the same DTC. Below is a table containing the most common manufacturers, models, and engine types that may experience a P20EE engine code: 

Manufacturer	Models	Engine
Cadillac	Escalade	Duramax turbo-diesel
Chevrolet	SilveradoExpress Cargo VanSuburban Tahoe	Duramax turbo-diesel
Ford	F-Series pickups	Power Stroke turbo-diesel
GMC	SierraSierra DenaliYukon	Duramax turbo-diesel
Jeep	GladiatorWrangler	EcoDiesel turbo-diesel
Mercedes-Benz	Sprinter Cargo VansSprinter Crew VansSprinter 2500 Passenger Van	BlueTEC turbo-diesel
RAM	15002500, 3500ProMaster Cargo VanProMaster 2500 Window Van	Cummins EcoDiesel turbo-diesel

A P20EE engine code typically illuminates the check engine light (CEL) to alert drivers to the issue. In addition, this engine code can also be accompanied by excessive black smoke coming from the tailpipe, poor performance, reduced efficiency, and an inability to pass emissions. Aftermarket or high-performance exhaust systems can also cause the P20EE DTC. 

Common Causes of P20EE Code

NOx sensor failure, SCR system issues, or DEF system problems can trigger a P20EE code. Leaks or damage to the vehicle’s exhaust system can also result in faulty exhaust stream readings that prompt this engine code. Let’s dive further into the common causes of the P20EE DTC.

NOx Sensor Failure

In addition to oxygen (O2) and temperature sensors, NOx sensors monitor the SCR system’s efficiency in modulating the exhaust stream’s temperature and NOx content. 

Faults in any of these sensors can trigger the P20EE code, but failure of the NOx sensor is a good starting point unless otherwise indicated. 

Verification of the NOx sensor’s readings will determine whether another issue is at play or the NOx sensor itself has failed. 

SCR Catalyst Issues

The SCR system on diesel engines reduces NOx in the exhaust stream. Fed by data from the temperature, O2, and NOx sensors, it alerts the powertrain control module (PCM) when to inject DEF to reduce NOx. 

When a NOx sensor isn’t the culprit for a P20EE engine code, the larger SCR system could be at fault. 

DEF System Problems

Among the many factors affecting a diesel powertrain’s catalyst efficiency, the DEF system is one of the more manageable to diagnose. 

Similar to exhaust gas recirculation (EGR) systems on gas-powered vehicles, the DEF system burns off remaining particles in the exhaust stream to control emissions. 

Diagnosing the potential cause of a P20EE in the DEF system requires careful inspection of the relevant components. 

DEF systems require clean fluid that’s specifically designed for that particular system, which sprays from a DEF injector into the exhaust stream. If the fluid is of the wrong type, contaminated, insufficient in amount, or unable to effectively exit the injector, a system error can occur. Leaks in the DEF system, — typically identified by a white residue, — can also trigger a P20EE code. 

Tools Required for P20EE Diagnosis

Identifying the root cause of the P20EE requires standard diagnostic tools such as an OBD-II scanner, a multimeter, and several advanced diagnostic tools. 

The complete tool list required for P20EE diagnosis depends on the extent to which you’re unable to resolve the underlying problem. Experience also plays a significant role. 

OBD-II Scanner

Scanning for the P20EE engine code and any related codes requires a standard OBD-II scanner. This tool allows you to learn more about what’s going on with the vehicle as the data logger understands it. Capturing live data can also help inform the diagnosis and repair. 

Multimeter

Since diagnosing the P20EE code requires testing electrical components, a multimeter will come in handy. With the multimeter, you can eliminate the O2, temperature, and NOx sensors or determine that they’re malfunctioning and contributing to the P20EE code. A multimeter will also tell you if a DEF injector receives the signal to fire and responds accordingly.

Advanced Diagnostic Tools

An infrared temperature gun allows you to measure the exhaust stream’s temperature and verify the readings against the system’s measurements. A smoke machine helps detect exhaust leaks. Additional advanced tools may be required, depending on the root cause of the P20EE engine code. 

What Are the Symptoms of P20EE?

P20EE rarely announces itself the way a mechanical failure does. Most of what a driver notices comes from the inducement strategy the manufacturer built in to force the repair, which means the symptom list often says more about how long the code has been ignored than about what actually failed.

Check Engine Light

For most of these trucks, the light is the whole complaint. The module reaches P20EE by calculation, so it requires a valid monitoring period at steady load before determining that the catalyst is underperforming. This usually means highway miles, and the light often shows up on a long haul before it ever does around town. The truck drives the same as it did that morning. Pull a freeze frame before clearing anything.

Reduced Engine Performance

A degraded catalyst does not cost horsepower on its own. Power loss on a P20EE truck comes from the inducement strategy pulling torque to force the repair, and sometimes from the calibration shifting NOx control back into the cylinder with heavier EGR and retarded timing. 

Customers tend to describe the truck as flat rather than broken. Ask when it started, because a derate that arrived days after the light usually means the countdown reached its next stage.

DEF or Emissions Warning

Diesel trucks carry a second channel of warnings in the driver information center, and the exact wording matters. Service Emission System, DEF Quality Poor, and a mileage countdown each send you down a different path, so have the customer read the message out word for word. 

Check whether P207F or P2201 is stored alongside. White crystalline residue around a fitting is worth a look too, since dried DEF marks a leak that has been there a while.

Reduced Fuel Economy

Modern diesels are tuned to burn efficiently and clean up NOx downstream, which is the whole point of carrying an SCR system. 

When that system stops holding up its end, the engine tends to lean harder on in-cylinder NOx control, and heavier EGR with retarded timing costs fuel. 

Soot loading climbs as well, which brings regens around more often. An owner reporting a measurable mpg drop is describing whatever set the code in the first place.

Limp Mode or Speed Limitation

This is the symptom that puts a truck on a hook. Inducement escalates in stages. A warning arrives with a mileage countdown, then a speed cap around 65 mph, then a lower cap, and on some platforms a no-restart condition once the counter runs out. 

Clearing the code does not send that counter back to the top. EPA issued guidance asking manufacturers to soften these derates through software updates, so check for a calibration before quoting the job.

Step-By-Step Diagnostics 

As you encounter the P20EE code, you’ll gain a sense of the underlying cause. Until then, follow the instructions below to determine the source of the P20EE DTC. Work through them in order, because each step either clears a suspect or hands you the fault, and the catalyst should be the last thing you judge.

1. Scanning and Confirming the P20EE Code

Connect the OBD-II scanner to the vehicle and retrieve the P20EE code. Gather as much information as possible at this stage to determine what other factors may be at play. 

Record the freeze frame before you clear anything, since engine load, exhaust temperature, and DEF level at the time of the fault indicate whether the monitor ran under normal conditions. When the P20EE code is confirmed, move to the next step. 

2. Inspecting Related Components

Weed out the possible causes of a P20EE engine code by visually inspecting the wiring, connectors, and other components of the SCR system, including the DEF system and NOx sensors. 

Look specifically for white crystalline deposits on fittings and corrosion at connectors, both of which appear well before a component fails outright.

3. Checking for Other Codes That May Affect or Relate to P20EE

Additional related codes include: 

• P20EC
• P20ED
• P20EF

These codes relate to the same system. 

• P20EC is SCR NOx catalyst over temperature, P20ED is pre-catalyst over temperature, and P20EF is pre-catalyst efficiency below threshold. 
• P207F reports DEF quality below specification and should be resolved before anything else, since off-spec fluid is the most direct route to poor catalyst efficiency. 
• P2201 through P2209 cover the NOx sensors themselves.

4. Testing NOx Sensors

These sensors report through their own control unit, so there is no sensing element resistance to measure. 

Graph the upstream and downstream readings together on live data instead, where the downstream value should sit well below the upstream one once the system is dosing. 

Ensure that the readings fall within the manufacturer’s specifications. Testing the reductant control module and DEF injector can also help rule out the causes of the P20EE issue. A multimeter still earns its keep on the sensor heater circuits and supply voltage.

5. Inspecting DEF Fluid Quality and System Integrity

DEF quality and system integrity play an integral role in the SCR system. Inspect the fluid to determine if it’s low or contaminated. 

Concentration cannot be judged by eye, so confirm it with a refractometer against the 32.5 percent urea specification. 

Leaking DEF will also leave a white residue, so inspect components carefully to detect any leaks that could cause insufficient DEF. 

6. Verifying Exhaust and Emission System for Leaks or Damage

Inspect the exhaust and emissions systems for any obvious leaks or damage. Pay close attention to fittings or couplings. A smoke machine can also help you detect leaks not visible to the naked eye. 

Leaks matter here because air entering the gap between the two NOx sensors skews the comparison the monitor relies on. 

Check the exhaust gas temperature sensors while you are under there, as dosing stays suspended until the catalyst reaches operating temperature.

7. Checking for Bulletins and Calibration Updates

Run the VIN through the manufacturer’s system before ordering parts. GM, Ford, and Stellantis have all published bulletins attributing P20EE to software, and several of those remedies are a reprogram with no parts involved.

4 Steps to Fixing P20EE Code

After completing the diagnostic steps above, determine which components require attention, whether by repair or replacement. 

Note that you can perform many of these fixes with Direct-Hit Mobile at your fingertips without having to leave your work area to reference wiring diagrams or technical specifications. Direct-Hit Mobile gives you instant access from your mobile workstation, right in the palm of your hand. 

1. Address the Identified Root Cause

Throughout the diagnostic process, you should have identified potential repairs required to bring the NOx sensors, SCR system, and DEF system back in line. Complete those repairs, noting which components you worked on in case the issue is multifaceted. In the event there are multiple issues, it may be worthwhile to review the system after each significant repair to further diagnose any additional concerns. 

2. Reset the P20EE Code Using an OBD-II Scanner

Once you’ve repaired or replaced the necessary components to fix the underlying cause of the P20EE DTC, it’s time to reset the computer. Plug in your OBD-II scanner and clear the P20EE engine code. If the issue triggered other related codes, clear those as well. 

Clearing also resets the readiness monitors, so the truck will report not ready until they run again. On a derated truck, the inducement counter does not reset with the code, and restoring full power usually takes a bidirectional tool.

3. Conduct a Test Drive to Confirm the Repair

Test-drive the vehicle to determine whether the repair has resolved the issue. Driving with the OBD-II scanner still attached can help technicians catch any issues in real time as they arise. 

Once the test drive is complete, verify that the emissions systems and related components are in proper working order. 

The catalyst monitor needs steady load before it will run, and a lap around the block proves nothing. Watch the downstream NOx reading fall below the upstream one as the system doses, since that comparison is what the module itself makes.

4 . Verify That the Code Does Not Return

Continued use will determine if the underlying issue was addressed or if the problem still remains. In the best-case scenario, the P20EE code won’t reappear. If it does, further diagnosis and repairs may be required. An absent code is not the same as a passed monitor. Check that the SCR readiness status reports as complete rather than not ready, which matters most for any customer heading to an emissions test.

P20EE: Causes, Diagnosis, and Repair

P20EE is a verdict on system efficiency rather than a component fault, so the code names the catalyst without proving anything about it. Almost everything upstream can drag that calculation below threshold. Work from the cheap inputs toward the expensive part.

Common Causes

DEF quality sits at the top of the list, and a stored P207F alongside P20EE points straight at it. NOx sensor drift comes next, since the module judges the catalyst by comparing two of them. 

Exhaust leaks, a crystallized dosing injector, and an outdated PCM calibration complete the rest. The catalyst belongs at the bottom.

Diagnosis

Pull the freeze frame and scan every module, including the reductant controller, which a generic tool often misses. Test the fluid with a refractometer against the 32.5 percent urea spec. 

Then graph both NOx sensors under the conditions the OEM monitor uses, because a comparison taken at idle proves nothing. Smoke test the exhaust and check for bulletins.

Repair

Clear the contributing faults first. Refill with sealed DEF, fit an OE NOx sensor, seal any leaks, and load any calibration released by the manufacturer. Crystallized dosing hardware sometimes clears with a commanded regeneration. 

Verify with a completed drive cycle. A light that went out proves nothing on its own. Save the catalyst for last, because replacement runs into thousands.

P20EE Chevy: Causes, Diagnosis & Repair

Chevrolet’s diesel range runs from the 6.6L Duramax in Silverado HD down to the 3.0L in half-tons and full-size SUVs, and P20EE shows up across the board. GM has published enough bulletins about false detections that a calibration check should be at the front of the job.

Common Causes

Software leads on these trucks. GM bulletin 21-NA-128 attributes P20EE, P026C, and P11D5 on 2021 L5P models to a software anomaly and calls for an ECM reprogram, covering Sierra alongside Silverado. Beyond calibration, look at NOx sensor drift, DEF quality, and a carboned reductant injector before you consider the catalyst.

Diagnosis

Check the VIN for open bulletins and campaigns first, since a Silverado can arrive with a calibration already waiting. Confirm the fluid with a refractometer, then graph both NOx sensors through a steady highway pull. Exhaust conditions during an active regen look nothing like the monitor’s, so data captured mid-regen will mislead you.

Repair

Load the calibration before ordering parts, because a reflash resolves a fair share of these outright. Use ACDelco NOx sensors. Cheap replacements are a known source of repeat codes on the L5P. Finish with a completed drive cycle and check coverage before quoting, since bulletin work is often warranty-eligible.

P20EE GMC: Causes, Diagnosis & Repair

Sierra HD shares the 6.6L Duramax and its entire aftertreatment system with Silverado HD, so most GM bulletins on this code name both trucks. The differences worth knowing are in GMC’s model mix, where the Sierra Denali and 3.0L Yukon models bring their own patterns.

Common Causes

GM bulletin 22-NA-069 covers Sierra and Silverado 3500 4WD trucks with under 10,000 miles that set P11D5 or P20EE due to unusual NOx sensor learn values, linked to changes in altitude and humidity. 

On 3.0L Sierra and Yukon models, high estimated DEF consumption while towing produces warnings that resemble a fault. Sensor drift and fluid quality cover most of the rest.

Diagnosis

Run the VIN before anything else, because the remedy on a low-mileage Sierra is often a reductant control module reprogram with no parts involved. 

Ask whether the truck was towing when the warning appeared. Then verify fluid quality and compare NOx sensor data under load, away from any active regen.

Repair

Reprogramming the reductant control module resolves low-mileage learn-value cases without opening the exhaust. Where a sensor has actually drifted, fit an OE part and let the module relearn afterward. 

Fleet trucks often arrive with several emissions codes at once, so work them in order rather than clearing the lot and hoping.

P20EE Ford: Causes, Diagnosis & Repair

Ford’s 6.7L Power Stroke accounts for most P20EE traffic on the Super Duty side, and the pattern there is unusually consistent. The upstream NOx sensor and PCM calibration cover a large share of cases, which makes a bulletin search the highest-value ten minutes of the job.

Common Causes

A failing upstream NOx sensor feeding bad data is the usual root cause, and P2201 stored alongside P20EE points directly to it. 

Ford has published bulletins on those sensor codes calling for a replacement sensor plus a PCM reprogram, including one covering 2020 to 2022 Super Duty trucks. DEF quality and exhaust leaks fill the remainder.

Diagnosis

Pull the full code set before anything else, because the companion codes indicate which bulletin applies. Check the upstream sensor’s response against the downstream sensor’s response during a loaded highway run. Calibration sensitivity has resulted in false detections on these engines, so confirm the truck is on the current software.

Repair

The published remedy pairs a NOx sensor replacement with a PCM reprogram, and doing only one half tends to bring the truck back. Use Motorcraft sensors. Check warranty and campaign status before quoting, because emissions terms vary by weight class and model year, and work like this is frequently covered.

Use Identifix Tools to Fix Code P20EE

Successfully diagnosing and resolving a P20EE engine code requires an understanding of the components and systems involved. While these repair steps provide a foundation for diagnosing and addressing the P20EE code, it’s essential to note that this information is only a general guide. 

For the most precise and vehicle-specific instructions, including detailed wiring diagrams and manufacturer specifications, we recommend consulting the Direct Hit library. This comprehensive resource offers in-depth insights tailored to each vehicle’s make and model, helping professional mechanics achieve accurate, lasting repairs.

When it’s time to get under the hood or even under the vehicle, rely on Identifix Direct-Hit Mobile to find the fix you need faster from anywhere at any time. Direct-Hit Mobile offers technicians updated guides, a virtual technician hotline, and direct access to OE data, all without leaving the workspace. 

Contact us to learn more about integrating Direct-Hit Mobile into your auto repair shop to match speed and precision at the source.  

See Other Related P Code Articles

P0420 | P0300 | P0171 | P0128 | P0456 | P0442 | P0332 | P0352 | P0455 | P0113 | P0174 | P0301 | P0172 | P0011 | P0299 | P0700

FAQs

What does the P20EE code mean?

P20EE stands for SCR NOx Catalyst Efficiency Below Threshold on bank 1. The module compares the NOx sensor ahead of the catalyst with the one behind it and determines that the system is not reducing nitrogen oxides sufficiently. It only appears on diesels with selective catalytic reduction. The code describes a calculation, so it names the catalyst without proving the catalyst failed.

How long can I drive with P20EE?

Longer than the warning suggests, but not indefinitely. Nothing about the code itself damages the engine, and most trucks drive normally until the countdown starts. Once a mileage counter or a speed cap appears, the truck is on borrowed time, and the stages only escalate. Avoid towing until the fault is confirmed, and treat any speed limitation as a same-week repair.

What is the most common cause of P20EE?

DEF quality and NOx sensor problems account for most of them, with outdated calibration close behind. A stored P207F points at the fluid, and a P2201 points at the upstream sensor. The catalyst itself is the least likely cause on a truck with reasonable mileage, and replacing it first is the expensive mistake on this code.

How do I clear the P20EE code?

Any scan tool will erase it, though that alone solves nothing. Fix the underlying fault, then clear the code and run the drive cycle so the monitor completes and reports a pass. Clearing also resets readiness monitors, which matters if an inspection is coming. On a derated truck, the inducement counter does not reset with the code, so plan on a capable tool.

Why does P20EE keep coming back?

Usually because the repair addressed a symptom rather than the cause. Common repeats include a new catalyst behind a drifting sensor, fresh fluid poured into a system still holding contaminated DEF, an aftermarket NOx sensor the module never trusts, and a calibration that was never loaded. A small exhaust leak ahead of the rear sensor will do it too.