P2270: O2 Sensor Signal Biased/Stuck Lean Bank 1 Sensor 2
What to do
The car will usually drive normally — this sensor does not control fuelling, it monitors the catalytic converter. If a converter has been diagnosed alongside this code, do not agree to it yet. The converter verdict is calculated from this sensor, so a stuck one makes that judgement worthless in both directions: it can condemn a good converter or hide a dead one. Repair this first, have the monitor re-run, and see what the result is then.
What this code means
The post-catalyst sensor on bank 1 reports lean and will not report anything else.
The confusing part is that a downstream sensor is supposed to sit fairly still. Upstream, a healthy signal oscillates several times a second — that is the whole argument at P2195. Downstream of a working catalyst it should do almost the opposite: the converter stores and releases oxygen, which smooths the switching out, so a lazy, slow-moving rear signal is the sign of a healthy catalyst rather than a failing sensor. Comparing the two sensors' activity is exactly how the catalyst monitor works.
So "stuck" means something stronger here than it does upstream, and the module tests for it differently — it looks for the rear sensor failing to respond even when the mixture is deliberately driven well away from ideal, which a working sensor must still follow.
The money argument is the one this position always carries. The catalyst monitor is computed from this sensor, so a stuck one corrupts that judgement in whichever direction happens to apply:
- It can falsely condemn a good converter, because a rear signal that never moves is the signature the monitor looks for when a catalyst has stopped storing oxygen. A P0420 alongside this code is very probably not a failed converter.
- It can equally hide a genuinely dead one, if the stuck value happens to sit where the monitor expects a healthy catalyst to hold it.
Either way the converter verdict is unreliable until this sensor is right, and a converter costs many times what this sensor does. Fix this first and re-run the monitor before accepting any catalyst diagnosis.
The causes are those at P2195 with one addition specific to being underneath the car: this sensor and its harness live in the road spray, so wiring and connector faults account for a larger share here than they do upstream.
Likely causes
Start here
- Faulty oxygen sensor · moderate
- Damaged wiring harness · moderate
- Exhaust leak before the sensor · moderate
Less common
- Contaminated oxygen sensor element · moderate
- Corroded or loose connector · easy
- Faulty oxygen sensor heater · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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