What this code means
P20EE may mean the SCR NOx catalyst efficiency is being judged lower than expected by the engine control system.
What the vehicle may do
- The vehicle may display an emissions or service warning.
- The vehicle may have aftertreatment-related driveability or warning behavior.
- The code can often require a complete SCR and NOx sensor diagnostic path rather than a single-part conclusion.
Possible fault areas
- Possible DEF quality or contamination concerns.
- Possible reductant pressure, delivery, injector, or line issues.
- Possible exhaust leaks or aftertreatment sealing concerns.
- Possible NOx sensor feedback issues.
- Possible intake, charge-air, fuel, oil, coolant, or engine mechanical influences.
- Possible SCR catalyst efficiency concern.
Diagnostic path
Open on what P20EE is looking at
On this Silverado diesel, P20EE is an SCR NOx catalyst efficiency code. In plain terms, the control module may be seeing that the aftertreatment system is not reducing NOx as well as expected. The truck may show an emissions warning, may request service, and can be affected by possible DEF quality or contamination issues, reductant pressure or delivery problems, exhaust leaks, NOx sensor feedback problems, intake or charge-air leaks, engine condition issues, or the SCR catalyst itself. Before getting deep into parts, remember how this system works: reductant is metered into the exhaust ahead of the SCR catalyst, and the upstream and downstream NOx sensors let the module compare engine-out NOx against tailpipe-out NOx. For P20EE, the important diagnostic idea is simple: the measured NOx conversion efficiency is lower than expected.
Start with the basic checks and code triage
Start with the basic system checks, then follow a structured diagnostic approach before using the P20EE path. Do not skip the upfront code triage. If other prerequisite codes are present, check what they mean first and handle those before judging SCR efficiency. After those are handled, run the scan tool Reductant System Malfunction Warning Service Bay Test. If that test passes, the system is considered All OK at that point. If it does not pass, move into the system testing. If there are no other codes and P20EE is not currently set, that is also All OK. If P20EE is set, continue with testing. The monitor itself is gated; for P20EE, the engine is running, barometric pressure is greater than 70 kPa (10.2 psi), ambient air temperature is greater than -7°C (19.4°F), the reductant tank is not frozen, and the vehicle is not in a DPF regeneration.
Inspect the aftertreatment hardware and DEF condition first
Once you are in system testing, start with the physical checks. Look over the diesel oxidation catalyst, the NOx sensors, the SCR system, and the diesel particulate filter for damage, alterations, catalyst substrate rattle, exhaust leaks, and loose NOx or exhaust temperature sensors. If you find one of those problems, repair it as needed before continuing. If that checks out, test the reductant system for contamination. Water in the reductant tank can set these codes, so DEF condition matters here. If contamination is found, correct that condition. If no contamination is found, command the Reductant System Leak Test on with the scan tool.
Check reductant pressure and pump response
During the leak test, verify the system can build the Reductant Pressure parameter greater than 450 kPa. If pressure is low, command the Reductant Pump Duty Cycle Command Device Control to 50% with the scan tool, then compare Desired Reductant Pump Speed to actual Reductant Pump Speed. If desired and actual pump speed are not equal, pause this efficiency diagnosis and work the pump control fault path first; do not guess past that split. If desired and actual speed are equal while pressure is still below the specified range, replace the A38 Reductant Pump and Sensor Assembly. If reductant pressure is within range, move on and check the reductant lines for leaks, kinks, or restrictions. Repair any line problem found.
Verify dosing, deposits, engine inputs, intake, and exhaust sealing
Next, verify the Reductant Fluid Injector Quantity Test passes. If it does not pass, repair the cause before continuing. When using injector control, a ticking sound should be heard at the reductant injector. If the quantity test passes, inspect the exhaust for reductant deposits and remove deposits if they are present. Then check for conditions that can skew the NOx efficiency result: a leaking aftertreatment fuel injector, leaking engine fuel injectors, oil consumption, coolant consumption, or an engine mechanical condition. If one of those exists, repair it as needed. After that, verify there are no leaks or restrictions in the intake system or charge air cooler hoses. Then pressurize the exhaust system and check from the warm up nitrogen oxides catalytic converter forward with soapy water. Pay close attention at the reductant injector mounting surface, around the NOx sensors, and around the exhaust gas temperature sensors. No exhaust leaks should be found, although a small amount of very small bubbles is acceptable.
Run the NOx sensor comparison checks
If the exhaust leak check is clean, disconnect the electrical connector at the Q61 Reductant Injector. Handle the road test safely. Drive until NOx Sensor 1 Heater Mode and NOx Sensor 2 Heater Mode display On. Then, with the transmission in park, let the engine idle for 10 minutes. At that point, both NOx sensor concentration readings should be greater than 80 ppm, they should be within 50 ppm of each other, and NOx Sensor 2 should not be more than 30 ppm greater than NOx Sensor 1. If those concentration conditions are not met, reconnect the reductant injector, drive the vehicle, and perform a 3-5 second full throttle acceleration followed by a closed throttle deceleration. During deceleration, NOx Sensor 1 Concentration should be less than 20 ppm. If it is greater than 20 ppm, replace the B195A Nitrogen Oxides Sensor 1. If NOx Sensor 1 passes that check, then verify NOx Sensor 2 Concentration is 50% or less of its peak value during acceleration. If NOx Sensor 2 is greater than 50% of that peak value, replace the B195B Nitrogen Oxides Sensor 2. If that response is acceptable, return to the service bay test portion of the diagnostic path and avoid guessing outside the earlier checks.
Reset loading and prove the system with the service bay test
If the NOx sensor concentration conditions are met during the injector-disconnected idle check, reconnect the Q61 Reductant Injector and perform the scan tool NOx Catalyst Reductant Loading Reset. Then perform the Reductant System Malfunction Warning Service Bay Test and verify it passes. If it passes, the result is All OK. If it does not pass, command DPF Regeneration Enable on with the scan tool, drive the vehicle safely until regeneration completes, and run the Reductant System Malfunction Warning Service Bay Test again. If the test passes after regeneration, the result is All OK. If it still does not pass after regeneration, replace the warm up nitrogen oxides converter.
Verify the repair and close
Keep verification separate from the testing path. After the repair, verify the repair and confirm the code stays gone by performing the scan tool Reductant System Malfunction Warning Service Bay Test. Before returning the vehicle to the customer, use that test to verify proper reductant system operation. If no repairs were performed, that service bay test is not necessary. If the verification test does not pass, go back into system testing. If it passes, the result is All OK. The takeaway: P20EE is not just a catalyst call. Work the code in order: prerequisites, reductant pressure and dosing, contamination, leaks, engine and air handling issues, NOx sensor behavior, then final service bay confirmation. For more diagnostic training, visit stepdiagnostics.com.
Final check
P20EE can be approached as an SCR efficiency problem that must be separated from dosing, sensor, leak, contamination, and engine-condition influences before judging the catalyst.
For more guided automotive diagnostics, visit STEP Diagnostics.
