What it means
The control module detects that the Bank 1 Sensor 2 NOx signal is biased low or remains low when it should change. The code identifies a signal-performance fault, not a specific cause.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The Bank 1 Sensor 2 NOx sensor measures nitrogen oxides in the exhaust downstream of the first sensor on bank 1, helping the control module assess emissions-control performance. The module sets P225F when that sensor's signal stays lower than expected or fails to rise and change as exhaust conditions change.
Common causes
Most likely first.
- Sensor aged
- Sensor element aged beyond calibration window
- Sensor element contaminated by oil, soot or coolant
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
How to diagnose it
- Read freeze-frame data and live NOx readings for Bank 1 Sensor 2, comparing them with the upstream sensor and operating conditions to confirm the low or unchanging signal.
- Inspect the Bank 1 Sensor 2 wiring, connector, and exhaust area for heat damage, corrosion, loose pins, or contamination.
- Check for oil, soot, or coolant contamination affecting the sensor element and consider whether an engine or exhaust fault is causing it.
- Use a scan tool to observe whether the downstream NOx reading responds as conditions change, following the vehicle's service information.
- Test the sensor circuit and sensor operation using the manufacturer's diagnostic procedure before replacing the sensor.
Common fixes
- Repair damaged wiring or secure a loose or corroded Bank 1 Sensor 2 connector.
- Address any oil, coolant, or excess soot source contaminating the sensor.
- Replace the aged or contaminated Bank 1 Sensor 2 NOx sensor if tests confirm it is out of range.
Can I keep driving?
The vehicle may remain drivable, but emissions can rise and a NOx or DEF warning may appear. On vehicles with a countdown strategy, continued driving with the fault may eventually lead to a no-start restriction, so arrange diagnosis promptly.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.