P20EE — SCR NOx Catalyst Efficiency Below Threshold (Bank 1)
SCR NOx catalyst efficiency below threshold (Bank 1).
P20EE is logged when the engine ECU compares the upstream NOx sensor signal with the downstream sensor and concludes the SCR catalyst is no longer reducing NOx by enough to meet emissions limits. It is one of the most common Euro 6 diesel fault codes and almost always triggers the AdBlue warning and countdown.
P20EE symptoms
- AdBlue / Check Engine warning light
- 'AdBlue — no engine start in X miles' countdown
- Reduced power / limp mode under load
- Increased AdBlue consumption
- Failed emissions retest
Common causes of P20EE
Failed downstream NOx sensor
The downstream sensor degrades and reports artificially high NOx, making the SCR look inefficient even when it is working correctly. Most common cause.
Genuinely degraded SCR catalyst
Thermal damage, sulphur contamination or oil ingress destroys the catalyst's reducing capacity.
AdBlue dosing under-delivery
A crystallised injector or weak pump means insufficient AdBlue reaches the catalyst.
Wrong-spec or contaminated AdBlue
Off-spec urea destroys SCR substrate and gives a permanent P20EE.
Exhaust leak between sensors
Air ingress upstream of the downstream sensor skews the efficiency calculation.
Vehicles affected by P20EE
- • Mercedes-Benz Sprinter, Vito, C-Class, E-Class (OM651 / OM642)
- • BMW X5, 5 Series, 3 Series, X3 (N57 / B47)
- • Audi A6, A4, Q5, Q7 (3.0 TDI)
- • VW Crafter, Transporter, Touareg, Tiguan (2.0 / 3.0 TDI)
- • Ford Transit, Ranger, Kuga, Mondeo (2.0 EcoBlue / 3.2 TDCi)
- • Land Rover Discovery, Range Rover (TDV6 / Ingenium)
- • Peugeot, Citroen, Vauxhall, Renault vans (BlueHDi / dCi)
P20EE diagnostic process
- Manufacturer-level scan (Xentry / VCDS / IDS / DiagBox)
- Live data: upstream and downstream NOx mV / ppm, SCR temp, dosing duty cycle
- Pressure-test the AdBlue dosing line and verify injector spray
- Confirm AdBlue fluid quality and tank sender accuracy
- Smoke / pressure-test exhaust between SCR and downstream sensor
Common P20EE repairs
- Replace downstream NOx sensor with OE-coded part
- Replace AdBlue dosing injector / clear crystallisation
- Replace SCR catalyst where genuinely failed
- Drain and refill with OE-spec AdBlue after contamination
- Reset SCR adaptation values and clear stored codes
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