P0142 indicates that the engine control module has detected an electrical fault in the circuit for the third oxygen sensor on bank 1, typically the post‑catalyst sensor. The code is set when the sensor’s signal voltage is out of the expected range or the heater circuit is not functioning.
The sensor itself may have an open or shorted element, or its internal heater may be inoperative, causing the circuit voltage to fall outside limits.
Corroded, frayed, or disconnected wires and pins in the sensor harness can create an open or short circuit.
A leak upstream of the sensor allows excess oxygen to reach the element, skewing its voltage output and triggering the code.
A blown fuse or defective relay supplying power to the sensor heater prevents the heater from reaching operating temperature.
An internal driver or reference voltage problem in the control module can mimic a sensor circuit fault, but this is uncommon.
Almost never. The code is almost always caused by the sensor, its wiring, an exhaust leak, or the heater circuit. PCM replacement is only considered after all other possibilities have been ruled out.
A failing catalyst can affect downstream sensor readings, but P0142 specifically points to a circuit fault rather than a performance issue. An exhaust leak caused by a cracked catalyst could trigger the code.
Clearing the code only turns off the light temporarily. If the underlying circuit fault remains, the code will return once the monitor runs again.
The vehicle will typically remain drivable, but fuel economy may suffer and emissions will be higher. Extended operation with a faulty downstream sensor can also mask catalyst deterioration.