Emissions

P20EE — SCR NOx Catalyst Efficiency Below Threshold

Severity: medium Drive with care, to a shop

What P20EE means

The SCR system sprays diesel exhaust fluid (DEF, AdBlue) into the exhaust, where the catalyst converts nitrogen oxides into nitrogen and water. The control unit compares NOx sensor readings before and after the catalyst. If the drop is smaller than expected, it sets P20EE.

Symptoms

  • Check engine light on
  • DEF/AdBlue or SCR warning message
  • Reduced power or torque limitation
  • Countdown to a no-start condition
  • Unusually high or low DEF consumption

Possible causes

  1. Contaminated, diluted or old DEF
  2. Worn or damaged SCR catalyst
  3. Clogged or crystallized DEF dosing injector
  4. Faulty NOx sensor
  5. Failed dosing valve or supply module
  6. Frozen or crystallized DEF lines, or failed line heaters
  7. Damaged wiring or corroded connectors
  8. Low DEF level

How to diagnose

  1. Scan all codes and freeze-frame data, looking for other SCR or NOx sensor codes
  2. Check DEF level and test its concentration with a refractometer
  3. Inspect the tank, lines and dosing injector for leaks and white urea crystals
  4. Verify that the line and supply module heaters work
  5. Compare upstream and downstream NOx sensor readings at operating temperature
  6. Test dosing pressure and quantity with a scan tool
  7. Inspect SCR wiring and connectors, checking power and ground
  8. Look up OEM service bulletins and ECU software updates

Common fixes

  • Drain bad DEF, flush the system and refill with fresh fluid
  • Clean or replace the dosing injector
  • Replace a faulty NOx sensor
  • Replace the supply module or dosing valve
  • Replace the SCR catalyst if it tests bad
  • Repair wiring faults and update ECU software

Questions and answers

Can I drive with P20EE?

Briefly, yes, but the engine may be derated and later may not restart after a countdown. Diagnose it soon.

Will changing the DEF fix it?

If the fluid is diluted or old, often yes. Clear the code after refilling and see whether it returns.

Do I need a new SCR catalyst?

Not usually. It is the costliest and least common cause, so check DEF, the injector, NOx sensors and wiring first.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.