What it means
The fourth oxygen sensor on bank 1, where present, returns implausible values during long-term plausibility evaluation. Used on multi-stage aftertreatment systems.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Bank 1 Sensor 4 is a fourth oxygen sensor on the side containing cylinder 1, fitted only on some multi-stage exhaust aftertreatment systems. The control module monitors its downstream signal over time for plausible behavior. This code sets when the long-term signal falls outside the expected performance window, which can result from sensor aging or a loose mounting that weakens or damps the reading.
Common causes
Most likely first.
- Sensor aging
- Sensor element aged beyond calibration window
- Sensor mounting loose - signal damping
Symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
How to diagnose it
- Save freeze-frame data and confirm the vehicle is equipped with Bank 1 Sensor 4, since many vehicles do not use a fourth sensor.
- Check live data from Bank 1 Sensor 4 under the manufacturer's stated monitoring conditions and identify whether the signal is consistently outside the expected range.
- Inspect the fourth sensor's mounting and wiring for looseness, heat damage, corrosion, or a poor connector fit.
- Check the exhaust section ahead of Bank 1 Sensor 4 for leaks that could alter the measured oxygen.
- If the mounting and circuit are sound, test sensor response against the manufacturer's specification.
Common fixes
- Secure a loose Bank 1 Sensor 4 mounting.
- Repair damaged wiring or clean and secure its connector.
- Repair an exhaust leak upstream of the sensor.
- Replace Bank 1 Sensor 4 if it fails the specified response test.
Can I keep driving?
If the vehicle runs normally, short trips are generally possible while arranging diagnosis. The fault can increase emissions and lead to an emissions-test failure; investigate promptly if rough running develops.
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.