P2243: O2 Sensor Reference Voltage Circuit/Open Bank 1 Sensor 1
What to do
Expect worse fuel economy and reduced power while the module fuels without a valid mixture measurement. If the sensor is replaced, insist on the correct part number for the exact engine rather than one that simply fits — these sensors carry an individual calibration in the connector, so a close-enough part reads plausibly and wrongly without ever setting a code. For the same reason, do not let a damaged connector be repaired by transplanting one from another sensor. Ask whether any other sensor is reporting a reference fault too; a shared supply failing looks like several faulty parts and is one.
What this code means
An open circuit in the reference voltage supplied to the upstream sensor on bank 1.
A wideband sensor needs a reference because its measurement is a comparison, not a reading. The pump cell described at P2237 is driven until a small sensing cell inside reports that the measurement chamber sits exactly at the ideal mixture, and "exactly at the ideal mixture" is defined by a fixed reference voltage the module supplies. Lose the reference and the sensor is not merely inaccurate — the target it is being driven towards no longer exists, so the pump current means nothing.
The detail worth knowing before any part is bought is the calibration resistor. Manufacturing tolerance on these cells is wider than the accuracy the fuelling strategy needs, so many wideband sensors are individually measured at the factory and carry a trim resistor moulded into the connector body to record the result. The module reads it and corrects. Three consequences follow, and all three are met in workshops:
- Part numbers are not interchangeable even when the plug fits. A sensor from a related engine can install perfectly, read plausibly, and be wrong by a margin that never sets a code — just a mixture quietly offset for the life of the car.
- The connector belongs to the sensor. Repairing a damaged plug by transplanting one from another sensor, or splicing a new sensor onto an old connector, breaks the pairing and the correction goes with it.
- A reference fault can be the resistor circuit rather than the cell. Which is worth establishing, because it can point at the connector rather than the sensor.
And the corpus's most repeated argument applies here in its original form: a reference voltage often feeds more than one thing. Where it is shared, a single failed supply produces faults on several sensors at once and none of them is faulty. That is the P0641 case exactly, and it is the reason to check what else is reporting before condemning anything.
Likely causes
Start here
- Open circuit in sensor wiring · moderate
- Corroded or loose connector · easy
- Failed sensor reference voltage supply · moderate
Less common
- Faulty oxygen sensor · moderate
- Incorrect oxygen sensor fitted · moderate
- Damaged wiring harness · 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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