If you’re seeing an AdBlue warning and countdown on a VW, Audi, SEAT, or Škoda in your workshop, there’s a good chance you’re dealing with fault code P20EE – NOx catalyst efficiency too low.
It’s one of the most common SCR-related codes on VAG group vehicles, and getting to the root cause quickly means understanding how the system works.
How the AdBlue system works
Selective Catalytic Reduction (SCR) reduces nitrogen oxides in exhaust gas
AdBlue travels from the tank through a plastic pipe to the injector, where it’s turned into fluid jets in the exhaust stream. These hit a mixer plate, turning into a fine mist that combines with the exhaust gas before passing through the catalyst, where NOx is reduced. The tank itself houses the AdBlue, pump, level sensor, heater, and temperature sensor.
Working through the diagnosis
Before touching a spanner, talk to the customer using open ended questions, the “five W’s” (what, when, where, who, why), to establish whether the vehicle has been to another workshop or had the AdBlue system mapped out, and check for existing technical service bulletins or TPIs.
From there, the diagnostic sequence is straightforward:
Check AdBlue quality with a refractometer; it should read around 32% strength. A cheap tool (from about £30) can save hours of guesswork; poor quality or contaminated AdBlue will trigger this code every time. (screen wash has been known to end up in tanks)
Check live data for the NOx sensors and AdBlue system pressure. For this vehicle needs to be at running temperature, since sensors don’t activate until exhaust temperature sensor 4 reaches roughly 200°C; before that, you’ll see a default reading of around 65,000 ppm. Pressure in the reductant line should sit at 5–6 bar when the pump is running, and zero when it isn’t.
Check that the system is operating correctly with a dosage test on the injector, confirming a strong, well-atomised spray rather than a mist, and checking for leaks or nozzle blockage.
Finally, clear the fault memory, road test with plenty of overrun driving (typically 50 miles), and confirm the countdown clears and no new codes are logged.
Common culprits
In practice, most Audi AdBlue faults trace back to the tank sender unit. These are no longer available separately; they come as part of a new tank, which requires a modified ECU coded via ODIS with the correct SVM software version code. Because of this, replacing both the tank and ECU together is usually the more reliable long-term fix.
Wiring damage from chafing on under-trays and internal leaks within the sender unit are also worth checking.
On Transporter models, two TPIs cover this fault specifically: one for an ECU software update, and a second used when the code returns after updating, covering fluid quality, EGR and glow plug checks, and a full learn-value reset.
Getting this fault right the first time comes down to method: question the customer properly, follow the system checks in order, and don’t skip the TPIs. Done this way, P20EE is one of the more predictable SCR faults to resolve
For more information on fault code P20EE, please see our webinar on this subject at: https://www.youtube.com/watch?v=U6qqLqoP9xc