OBD-II · Engine and powertrain (P)
P20EE
SCR NOx catalyst efficiency below threshold bank 1
What it means
The AdBlue (SCR) catalyst is not removing enough nitrogen oxides (NOx), judged by comparing the NOx sensors before and after it. On vehicles with SCR, a confirmed unresolved fault may trigger reduced performance or a no-restart countdown; follow the displayed warning and the vehicle handbook.
Symptoms
- engine warning light and/or AdBlue/emissions warning
- a no-start countdown in km on many cars
- usually no change in how the car drives
Likely causes, most likely first
| No. | Cause |
|---|---|
| 01 | poor-quality, diluted or old AdBlue |
| 02 | AdBlue dosing injector blocked by crystals or not spraying correctly |
| 03 | faulty NOx sensor (before or after the catalyst) |
| 04 | AdBlue pump or line problem giving low dosing |
| 05 | exhaust leak near the NOx sensors |
| 06 | aged or contaminated SCR catalyst |
| 07 | high engine-out NOx from an EGR fault |
What to check, step by step
- 01Read all SCR and NOx codes and live data (NOx before/after catalyst, dosing quantity).
- 02Test AdBlue concentration and inspect for contamination. If quality is outside specification, drain and service the system as prescribed and refill with ISO 22241 fluid; topping up does not reliably correct contaminated fluid.
- 03Remove and inspect the dosing injector for crystals; run a dosing quantity test if the tool supports it.
- 04Check for exhaust leaks and check both NOx sensors.
- 05Check EGR operation.
- 06Only then suspect the SCR catalyst itself.