Common among diesel engines, the P20EE Diagnostic Trouble Code (DTC) points to a fault with the nitrogen oxide (NOx) catalyst efficiency or Selective Catalytic Reduction (SCR) system. This comprehensive guide on diagnosing and resolving the P20EE engine code issue offers auto technicians valuable information regarding common causes, step-by-step diagnostic procedures, and the protocols necessary to complete the repair. Use the following instructions and troubleshooting tips to identify and fix the root cause of the P20EE code.
The DTC P20EE signals an issue with the diesel engine’s catalyst efficiency, which allows it to comply with emissions control regulations. In a nutshell, the SCR system monitors the amount of NOx within 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.
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.
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.
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 within any of these sensors can initiate the P20EE code, but failure of the NOx sensor can be a good starting point unless otherwise indicated. Verification of the NOx sensor’s readings will demonstrate whether there is another issue at play or if the NOx sensor itself has failed.
The SCR system on diesel engines oversees the reduction of NOx within 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.
Among the many components involved in a diesel powertrain’s catalyst efficiency, the DEF system is one of the more manageable components 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 with regard to the DEF system requires a careful inspection of the related 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 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 spark a P20EE code.
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.
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.
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.
An infrared temperature gun allows you to evaluate the temperature of the exhaust stream to verify the readings against the system’s measurements. A smoke machine assists with detecting exhaust leaks. Additional advanced tools may be required, depending on the root cause of the P20EE engine code.
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.
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. When the P20EE code is confirmed, move to the next step.
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.
Additional related codes include:
These codes relate to the same system. P20EC indicates an issue with the NOx sensor circuit. P20ED and P20EF relate to the DEF system, pointing to either contaminated DEF or a defective DEF tank temperature sensor.
Using your multimeter, test the NOx sensors for resistance. 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.
DEF quality and system integrity play an integral role in the SCR system. Inspect the fluid to determine if it’s low or contaminated. Leaking DEF will also leave a white residue, so inspect components carefully to detect any leaks that could cause insufficient DEF.
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.
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 the mobile workstation in the palm of your hand.
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 the event the issue is multi-faceted. 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.
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.
Test drive the vehicle to determine whether or not the repair has solved 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.
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.
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 serves only as a general foundation. 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 the make and model of each vehicle, 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 without having to leave 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.
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