P225E: NOx Sensor Performance - Signal Biased/Stuck High (Bank 1 Sensor 2)

P225E points at the downstream NOx sensor of bank 1, behind the SCR catalyst, reading persistently high. Because that sensor sees what is left after urea dosing, a high value can be a sensor offset or genuine NOx slipping through a weak catalyst, poor DEF or a faulty dosing valve. Volvo and Mack therefore put P225E in the group where either sensor or the aftertreatment itself may be the cause: their replacement guide and bulletin KC-107 require a NOx conversion test or the aftertreatment analysis in PTT before any part is changed, and for P225E they also send technicians to the DEF dosing valve diagnosis. No public bulletin from a passenger car maker covers P225E. P225C is the same fault upstream; P225F is the low-reading downstream case.

Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.

Symptoms

• MIL on, often together with P20EE
• Increased DEF consumption or DEF quality warnings
• Power reduction possible if SCR efficiency codes follow

Most likely causes

• Real NOx slip from a weak SCR catalyst, poor DEF quality or a faulty dosing valve
• Downstream NOx sensor with a positive offset
• Control software that evaluates the sensor incorrectly

What to check, in order

Start with the simplest. You can tick off each step.

1. Read the codes and the NOx data before and after the SCR catalyst
2. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
3. Check the sensor's connector and wiring
4. Look for exhaust leaks ahead of the sensor
5. Confirm that the AdBlue/DEF is of good quality
6. Replace the sensor only after ruling out power, ground, and leaks

Good to know

• Repair difficulty: hard
• It can trigger limp mode.
• It affects emissions.

What each character of P225E means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 2: Fuel injection
• 5E: Specific fault in that system

Frequently asked questions

Can I keep driving with code P225E?

You can usually keep driving in the short term, but get it repaired soon: on many diesels, the warning eventually limits starting or engine power.

What causes code P225E?

The most common causes are: real NOx slip from a weak SCR catalyst, poor DEF quality or a faulty dosing valve; downstream NOx sensor with a positive offset; control software that evaluates the sensor incorrectly.

Related codes

• P225FNOx Sensor Performance - Signal Biased/Stuck Low (Bank 1 Sensor 2)
• P225CNOx Sensor Performance - Signal Biased/Stuck High (Bank 1 Sensor 1)
• P221ANOx Sensor 1/2 Correlation (Bank 1)
• P20EESCR NOx Catalyst Efficiency Below Threshold (Bank 1)
• P229ENOx Sensor Circuit (Bank 1 Sensor 2)

Sources

• static.nhtsa.gov: MC 10188078 0001
• static.nhtsa.gov: MC 10201707 0001
• static.nhtsa.gov: MC 10217013 0001
• law.cornell.edu: 13 CCR 1968.2

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.