P2239 indicates that the Powertrain Control Module has detected a high voltage condition in the positive current control circuit for the upstream oxygen sensor on Bank 1, Sensor 1.
A sensor that has shorted internally can draw excessive current, causing the PCM to register a high voltage condition.
Open, shorted, or corroded wires in the positive current control circuit can create abnormal voltage levels.
A rare internal failure of the PCM’s driver transistor can produce a high‑current reading even with a good sensor and wiring.
Excessive voltage to the sensor’s heater element, often from a short to power, can trigger the high‑current code.
An inadequate ground can cause the sensor circuit to see higher than normal voltage, setting the code.
In most cases the PCM is not the culprit. The code is far more often caused by a bad sensor, wiring issue, or heater circuit problem. Replace the PCM only after confirming the sensor and wiring are functional and the PCM driver test fails.
No. A loose or faulty gas cap typically sets codes related to evaporative emissions (e.g., P0455) and does not affect the O2 sensor positive current control circuit.
Yes. The PCM will monitor the sensor circuit continuously; if the underlying fault remains, the code will reappear after a few drive cycles.
The vehicle will usually run, but you may experience reduced fuel economy, rough operation, and may fail emissions testing. Prolonged operation can cause the sensor to overheat, so repair is recommended promptly.