What P20EE means
P20EE stands for “SCR NOx Catalyst Efficiency Below Threshold.” The engine or aftertreatment control unit compares the upstream NOx sensor (before the SCR catalyst) with the downstream NOx sensor (after the catalyst). When the downstream value is too high relative to the upstream value for long enough, the monitor sets P20EE.
The code does not identify the failed part. It only reports that the system is not meeting its NOx conversion target. A complete diagnosis is needed before replacing the SCR catalyst.
P20EE symptoms
- Malfunction indicator lamp (MIL) and/or SCR system warning
- Active P20EE stored in engine or aftertreatment module
- Increased or abnormal DEF consumption
- Torque reduction, speed limiter or staged derate on trucks
- Failed emissions or roadworthiness test
Common causes of P20EE
| Cause | Why it reduces conversion |
|---|---|
| Contaminated or diluted DEF | Low urea concentration reduces available ammonia for the SCR reaction. |
| DEF doser fault | Incorrect quantity, poor atomization or leakage changes the NOx reduction rate. |
| DEF pump pressure issue | Low or unstable pressure prevents the doser from delivering the commanded dose. |
| Biased NOx sensor | A false reading can make conversion look worse than it really is. |
| Exhaust leak before SCR | False air changes NOx concentration and exhaust temperature. |
| Catalyst degradation | Thermal damage, oil/coolant contamination or sulphur poisoning reduce catalyst activity. |
Step-by-step P20EE diagnosis
- Read live data: Compare upstream and downstream NOx values at idle and under load. Both should respond; downstream should be clearly lower once dosing is active.
- Check exhaust temperature: SCR needs roughly 250–450 °C for efficient conversion. Low temperature from a failed thermostat, EGR issue or short trips can set P20EE.
- Inspect for exhaust leaks: Any leak upstream of the downstream NOx sensor invalidates the efficiency calculation.
- Test DEF quality: Use a refractometer. Urea concentration should be close to 32.5%.
- Test DEF system pressure and purge: Follow the OEM test to confirm the pump reaches and holds commanded pressure and completes its purge cycle.
- Inspect the DEF doser: Look for crystallisation, leakage, damaged spray pattern and correct electrical resistance.
- Validate both NOx sensors: Check supply, ground, bus communication and plausibility against known-good values.
- Evaluate the catalyst: Only after the above steps are correct, run the catalyst efficiency test and compare to OEM thresholds.
Parts to test before replacing the SCR catalyst
Replacing the SCR catalyst first is expensive and often unnecessary. Test these BG aftertreatment components first:
- NOx sensors — upstream and downstream references for 916+ applications
- DEF dosing modules / doser valves — 190+ references for stable DEF delivery
- DEF / urea pumps — complete pump assemblies and pressure solutions
- DEF pump repair parts — motors, diaphragms, filters and valves for remanufacturing
- SCR catalysts — for when the catalyst itself is confirmed failed
Frequently asked questions
What does P20EE mean?
P20EE is an OBD-II fault code indicating that the selective catalytic reduction (SCR) catalyst is not converting NOx efficiently. The downstream NOx sensor is reading too close to or above the upstream NOx sensor, so the monitor flags reduced catalyst performance.
Does P20EE mean the SCR catalyst is bad?
Not automatically. P20EE can be caused by exhaust leaks, contaminated DEF, a failing DEF doser, DEF pump pressure faults, biased NOx sensors or engine-out NOx that is outside the catalyst window. The catalyst should only be condemned after the supporting systems are proven correct.
What are the symptoms of P20EE?
Common symptoms include the malfunction indicator lamp, SCR warning, increased DEF consumption, torque reduction or derate on commercial vehicles, and a failed emissions inspection. Some vehicles show no drivability issue until derate begins.
How do you diagnose P20EE?
Start by reading freeze-frame data and live NOx values. Check for exhaust leaks, verify DEF quality and concentration, test DEF pump pressure and purge behaviour, inspect the doser for crystallisation or leakage, validate both NOx sensors, and confirm exhaust temperature reaches the SCR window. Only then evaluate catalyst efficiency against OEM thresholds.
Can you clear P20EE without fixing it?
A scan tool can clear the code temporarily, but the monitor will run again and the fault will return if the root cause is unresolved. Some emissions-related DTCs also have regulatory clearing requirements and may not reset until the prescribed monitor passes.