P0138 indicates the downstream (post-catalytic converter) oxygen sensor on Bank 1 is reporting a consistently high voltage above normal range, typically above 0.9 volts. This sensor should normally show low, fluctuating voltage after exhaust gases have passed through the catalytic converter. A sustained high voltage suggests the sensor is detecting persistently rich exhaust or has failed internally.
The sensor itself has degraded internal circuitry, contaminated heating element, or failed reference electrode causing it to read high voltage even when exhaust is not rich.
A leak in the exhaust pipe or muffler downstream of the sensor can allow outside air to enter, causing false high voltage readings.
A leaking fuel injector, faulty fuel pressure regulator, or evap system leak can cause overly rich air-fuel mixture that the post-cat sensor detects as high voltage.
Damaged wiring, corroded connector, or short-to-voltage in the O2 sensor circuit can cause artificially high sensor readings at the PCM.
Failed head gasket allowing coolant to enter combustion chambers creates rich running conditions detected by the sensor.
The engine computer may have a fault in its input circuit for this sensor, but this is infrequently the cause and should be diagnosed only after all other possibilities are ruled out.
In the majority of cases, yes. The downstream O2 sensor is the most common failure point for P0138. However, the underlying cause of rich exhaust or the code (such as an exhaust leak) should also be addressed to prevent recurrence.
Usually no. P0138 is a downstream sensor code; the post-cat sensor monitors catalyst efficiency but rarely causes converter failure on its own. The converter is only replaced if proven defective through catalyst efficiency testing.
It is rarely necessary. The PCM/ECM is an uncommon cause of P0138 and should be considered only after verifying the sensor, wiring, exhaust, and fuel system are all functioning correctly. Some shops may suggest PCM replacement because it is a simple solution, but this is usually incorrect for this code.
Yes, the vehicle will typically drive normally. However, you may fail emissions testing if required in your area, and the underlying condition (if rich exhaust) could cause long-term catalytic converter damage if not addressed.