P225F: NOx Sensor Performance - Signal Biased/Stuck Low (Bank 1 Sensor 2)

P225F is set when the downstream NOx sensor of bank 1 reports less NOx than expected or does not move away from a low value. A sensor behind the SCR catalyst that reads too low makes the catalyst look better than it is, so the efficiency monitor can no longer be trusted. The only public manufacturer source is the Volvo and Mack NOx sensor replacement guide for US10 and newer trucks: it lists P225F next to P225D among codes whose description mentions the NOx sensor although the sensor is rarely the cause, notes that the probe is reading fresh air, and tells technicians not to replace it. Leaks in the exhaust ahead of the sensor and harness faults are the more likely culprits. P225D is the upstream case; P225E the high-reading counterpart.

Medium severity. Can I keep driving?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.

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

Symptoms

MIL on
SCR efficiency cannot be confirmed, readiness may stay open

Most likely causes

Exhaust leak upstream of the downstream sensor drawing in outside air
Chafed or corroded wiring between sensor module and probe
Downstream NOx sensor with reduced sensitivity

What to check, in order

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

Read the codes and the NOx data before and after the SCR catalyst
With the connector unplugged, check whether the signal wire is making contact with ground.
Check the sensor's connector and wiring
Look for exhaust leaks ahead of the sensor
Confirm that the AdBlue/DEF is of good quality
Replace the sensor only after ruling out power, ground, and leaks

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P225F means

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

Frequently asked questions

Can I keep driving with code P225F?

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 P225F?

The most common causes are: exhaust leak upstream of the downstream sensor drawing in outside air; chafed or corroded wiring between sensor module and probe; downstream NOx sensor with reduced sensitivity.

Related codes

Sources

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.