P20EE: SCR NOx Catalyst Efficiency Below Threshold (Bank 1)

P20EE is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P20EE and illuminates the check engine light.

If your vehicle has triggered P20EE, you may notice one or more of these symptoms:

• Check engine light on
• AdBlue or emissions system warning, sometimes with a distance countdown before the engine can no longer be restarted
• Usually no change in how the car drives
• White crystal deposits around the AdBlue injector on the exhaust, where it is visible
• NOx sensor or AdBlue dosing codes stored alongside on some cars
• Can count against the car at an emissions inspection

Here are the most common causes of P20EE:

Generally yes, for short-term driving. The car usually drives normally, but do not ignore it. On many Euro 6 diesels a persistent SCR fault brings up an AdBlue or emissions warning with a distance countdown, and when the countdown reaches zero the engine will not restart after it is switched off until the fault is repaired. Get it diagnosed while the car still starts, and note the remaining distance if one is shown.

Follow these steps to pinpoint the root cause of P20EE:

1. Read all stored codes and the freeze frame. NOx sensor, AdBlue pressure, dosing or heater codes stored alongside usually explain the low efficiency and should be diagnosed first. A generic OBD2 reader shows the codes, the freeze frame and standard live data; some newer diesels also report NOx sensor values or the AdBlue level on standard readings, but on most cars the SCR data, the dosing tests and the adaptations need manufacturer-level software.
2. Check the AdBlue. It should meet ISO 22241 and come from a sealed container. If there is any doubt about what went into the tank (water, diesel or old fluid), have the concentration checked with a refractometer and the tank drained and refilled.
3. Inspect the AdBlue injector where it enters the exhaust, and the exhaust before the catalyst, for white crystal deposits and leaks.
4. With manufacturer-level software, compare the upstream and downstream NOx sensor readings and run the AdBlue dosing quantity test. A dosing test that delivers less fluid than specified points to the injector, the pump or the lines.
5. If dosing is correct and there are no leaks, check the NOx sensors against each other in the conditions the manufacturer specifies; a sensor that reads implausibly is replaced before the catalyst.
6. Only when the fluid, the injector, the sensors and the exhaust check out is the SCR catalyst itself the likely fault.
7. After the repair, clear the code. Many cars need the SCR adaptations reset with manufacturer-level software and a drive in the right conditions before the efficiency monitor runs again.

Repair costs for P20EE vary depending on the root cause and your vehicle. Here are typical estimates:

These codes are often seen alongside P20EE or indicate related issues: