What it means
The Bank 1 Sensor 1 NOx sensor responds too slowly to a high-to-low change. The code does not identify why the response is delayed.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Bank 1 Sensor 1 is the NOx sensor identified by the control system as Sensor 1 on Bank 1, typically upstream of the relevant aftertreatment component. The module monitors how quickly its NOx reading falls when conditions produce a high-to-low change. P24F4 sets when that specific sensor responds too slowly, which can happen as the sensor ages or its protective tube becomes soot-restricted.
Common causes
Most likely first.
- Aged sensor element with reduced response speed
- Soot clogging the protective tube
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 related NOx or reductant codes, confirming the fault names Bank 1 Sensor 1 and records a high-to-low response delay.
- Inspect the Bank 1 Sensor 1 connector and harness for heat damage, corrosion, loose pins, or chafing near the exhaust.
- Check the sensor's live NOx data and response during the manufacturer's prescribed operating conditions, comparing the high-to-low transition with expected behavior.
- Inspect the sensor's protective tube for soot restriction or physical damage if the manufacturer's procedure permits inspection.
- Check for exhaust leaks, dosing faults, or other aftertreatment problems that could keep NOx conditions from changing as expected.
- If wiring and exhaust conditions are sound, test or replace Bank 1 Sensor 1 according to the manufacturer's diagnostic procedure.
Common fixes
- Repair a damaged or loose Bank 1 Sensor 1 connector or harness.
- Repair an exhaust leak or related aftertreatment fault affecting the sensor's response conditions.
- Clean or service a soot-restricted protective tube only if permitted by the manufacturer.
- Replace the Bank 1 Sensor 1 NOx sensor if its response remains slow and the circuit checks pass.
Can I keep driving?
You may be able to drive if the vehicle runs normally, but have the emissions fault repaired promptly because NOx warnings, power limits, or a no-start countdown can follow. Avoid ignoring any displayed reductant countdown.
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.