P20EE: SCR NOx catalyst efficiency below threshold

Selective catalytic reduction converts nitrogen oxides into nitrogen and water, using ammonia released from injected urea solution. The ECU verifies it is working by comparing NOx before and after the catalyst.

P20EE means that comparison came up short.

Related: AdBlue and SCR system OBD diagnostics and P2200 to P2204 NOx sensor circuit.

A note on the code format

The letters are not a typo. P20EE uses hexadecimal digits, which extends the code range beyond what decimal digits allow.

Read it exactly like any other code: P1xxx manufacturer specific codes covers the numbering in general.

The conversion needs four things

All four must be present, and the code does not tell you which is missing.

1. AdBlue of the correct quality, in sufficient quantity.

2. Working dosing, delivering it into the exhaust as a fine spray.

3. Sufficient exhaust temperature — roughly above 200 °C. Below that, urea does not decompose into ammonia, and it can crystallise instead.

4. An intact catalyst with its coating still active.

The temperature condition catches people out

Worth stating plainly: a vehicle used for short urban journeys may never get the SCR system hot enough to work.

The system then reports poor conversion, and every component is healthy. The mismatch is between the vehicle and its use.

Worse, urea injected into an exhaust that is too cold crystallises, and those deposits block the dosing injector and coat the catalyst. Short-trip use therefore causes the failure as well as revealing it.

Short trips and engine damage.

Causes, ordered by cost

AdBlue quality

Contaminated or degraded fluid does not convert properly. AdBlue has a shelf life, degrades in heat, and is damaged by contamination — including by putting it in a container that held something else.

Buy from a source with turnover, and use fluid meeting the correct specification.

The dosing injector

Crystallised urea blocks it. Nothing reaches the catalyst, downstream NOx stays high, and the system reports poor efficiency.

Cleaning is often possible: P204F reductant system performance.

The NOx sensors

Two sensors, and either can be wrong. A downstream sensor reading high produces this code with a working catalyst.

Check that the sensors are plausible before condemning the catalyst — the catalyst is far more expensive: P2200 to P2204 NOx sensor circuit.

Excess NOx from the engine

The SCR system is sized for the engine’s expected output. An EGR fault producing more NOx than designed overwhelms it.

Fix the cause: P0400 and P0403 EGR flow and P0405 and P0406 EGR sensor.

The catalyst itself

Last. Coating degrades with age, with sulphur exposure, and rapidly if contaminated with oil or coolant.

The countdown

Emissions legislation requires that a vehicle with a defeated SCR system cannot be driven indefinitely.

So an SCR fault escalates: a warning with distance remaining, then reduced power, then no restart.

That is designed behaviour, not a malfunction, and it means this code has a deadline: AdBlue warning countdown.

What to log

Parameter	What to watch
NOx upstream and downstream	Should differ substantially
Exhaust gas temperature at the SCR	Above 200 °C for conversion
AdBlue dosing rate	Zero means dosing has stopped
AdBlue level	Obvious, and often overlooked
Distance in the countdown	Your timescale

Downstream NOx higher than upstream is physically impossible — that reading means a sensor is faulty, and it is a free diagnosis.

Do not delete it

SCR delete is illegal wherever it is mandated, fails inspection, and is detectable in the ECU calibration.

It also removes the only thing keeping NOx emissions near the legal figure — the pollutant with the clearest link to respiratory harm in urban air.

ECU remap and chip tuning detection and exhaust modification and emissions.

Step by step

1. Check AdBlue level and quality.
2. Check exhaust temperature reaches operating range on your journeys.
3. Check dosing rate — zero points at the injector.
4. Verify both NOx sensors are plausible.
5. Check for EGR faults producing excess NOx.
6. Consider the catalyst last.

Pair an ELM327 adapter, open the free tier of the app and log exhaust temperature at the SCR during a typical journey. If it never reaches 200 °C, the system was never given the chance to work.