P2200 to P2204: NOx sensor circuit on a modern diesel

Selective catalytic reduction converts nitrogen oxides into nitrogen and water using a urea solution — AdBlue — injected into the exhaust. Getting the dose right requires knowing how much NOx is actually present.

That is what NOx sensors do, and there are usually two: one before the SCR catalyst and one after.

Code	Meaning
P2200	NOx sensor circuit, bank 1
P2201	Range/performance
P2202	Circuit low
P2203	Circuit high
P2204	Circuit intermittent

Related: AdBlue and SCR system OBD diagnostics.

Three things worth knowing immediately

They are expensive. NOx sensors carry their own control electronics and are among the costliest sensors on the vehicle. That makes correct diagnosis worth more here than almost anywhere else.

They are consumables. Living in hot exhaust gas, they have a finite life. Failure at higher mileage is expected rather than exceptional.

Their failure stops dosing. Without a trustworthy NOx measurement, the ECU cannot meter AdBlue correctly, so it stops — and then the emissions system reports a fault, which starts a countdown.

The countdown is real

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

So the warning escalates:

1. A warning message with a distance remaining.
2. Reduced power.
3. No restart once the countdown reaches zero.

That is not a malfunction. It is designed behaviour, and it means a NOx sensor fault has a deadline attached.

AdBlue warning countdown.

Before replacing an expensive sensor

Check the cheaper explanations first.

AdBlue quality and level

Contaminated fluid, or fluid that has frozen and been degraded, prevents proper conversion. The downstream sensor then reports high NOx, and the system may blame itself.

Use fluid meeting the correct specification, from a source with turnover. AdBlue has a shelf life.

The dosing injector

A blocked or crystallised dosing injector delivers nothing, and NOx downstream stays high. That reads as a system fault while both sensors are healthy: P204F reductant system performance.

Crystallisation is common on vehicles doing short journeys where the exhaust never gets hot enough.

The SCR catalyst itself

A degraded catalyst cannot convert regardless of dosing: P20EE SCR NOx catalyst efficiency.

Wiring and connector

Underbody, hot, in the road spray. Standard checks, and worth doing before spending.

Upstream engine faults

High NOx production from an EGR fault overloads the SCR system. Fix the cause rather than the symptom: P0400 and P0403 EGR flow.

Reading the data

Log both NOx sensors, exhaust temperature and AdBlue dosing rate together.

Observation	Meaning
Upstream high, downstream low	System working
Both high	No conversion — dosing, catalyst or temperature
Downstream higher than upstream	Sensor fault, physically impossible otherwise
One sensor absent entirely	Circuit or sensor failure
Both plausible, dosing zero	Dosing system, not the sensors

SCR only works above roughly 200 °C. Short journeys where the exhaust never reaches that temperature produce high NOx with a perfectly healthy system — which is a usage problem, not a fault.

Short trips and engine damage.

Do not delete it

SCR delete is illegal in every market that mandates it, fails inspection, and is detectable in the ECU calibration.

It also removes the only thing preventing the vehicle emitting NOx at many times the legal level — which is the specific pollutant with the clearest link to respiratory harm in urban air.

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

In what order

1. Note the countdown, if one has started. It sets your timescale.
2. Check AdBlue level and quality.
3. Check the dosing system before the sensors.
4. Log both NOx sensors with exhaust temperature.
5. Check for EGR faults producing excess NOx upstream.
6. Inspect the connector and wiring.
7. Replace the sensor only when the rest is eliminated.

This check needs no workshop — an ELM327 adapter and the free tier of the app cover it. Log both NOx readings alongside exhaust temperature. A system that cannot get hot enough to work looks identical to a broken one until you check the temperature.