P241A: O2 Sensor Signals Swapped Bank 1 Sensor 1/Bank 1 Sensor 2
What to do
Mention any recent exhaust or sensor work when you book this in, including where it was done. This code describes two connectors being crossed, and it appears far more often after a repair than out of nowhere — so the history is genuinely the fastest route to the fix.
The repair itself is usually minutes rather than hours, since it is a matter of swapping two plugs back. Be wary of a quote for two new oxygen sensors before anyone has looked at which connector goes where.
Do not leave it for months. While the wiring is crossed the engine is being fuelled from the wrong sensor, which runs the catalytic converter hotter than it should and can turn a connector swap into a converter replacement.
What this code means
The engine control module has concluded that the two oxygen sensors on bank 1 are plugged into each other's connectors.
This is a wiring code that describes a specific mistake rather than a failure. The upstream sensor sits before the catalytic converter and swings rapidly as the fuelling is trimmed; the downstream sensor sits after it and should sit almost flat. When the module sees the flat trace on the channel that ought to be swinging, and the swinging trace where it expects calm, the only explanation that fits is that the two plugs have been crossed.
It is almost always set shortly after work on the exhaust. The two sensors on a bank often carry identical connectors, sit within reach of one another, and are replaced as a pair — which is exactly the situation in which they get put back the wrong way round.
Do not treat this as a sensor fault and replace them. Two new sensors fitted the same crossed way set the same code, which is how this one occasionally survives a repair.
The consequence is worse than a stored code, because the module is now trimming fuel on the wrong signal. It is reading a post-catalyst sensor as though it reported combustion, so the closed-loop correction it applies is based on gas that has already been through the converter. Expect the mixture to drift, the converter to run hot, and catalyst efficiency codes to follow if it is left.
Likely causes
Start here
Less common
- Incorrect oxygen sensor fitted · easy
- Incorrectly wired universal oxygen sensor · moderate
Rare
- Damaged wiring harness · moderate
Symptoms
Components involved
Information, not instruction
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