Is It Safe to Drive With a P0138 Code?
Common Symptoms of a P0138 Fault
EML on
May contribute to P0420
Possible rich running symptoms
Possible Causes of P0138
A P0138 code can be triggered by faults across several systems. Identifying the exact root cause requires proper diagnostic equipment and live data — not just reading the code.
O2 Sensor Wiring Short to Voltage
Downstream O2 sensor wiring shorted high.
O2 Sensor Failure
Downstream sensor stuck reporting rich.
Rich Exhaust Condition
Engine running rich causing genuinely high downstream O2 voltage.
Catalyst Failure
Failing catalyst causing abnormal downstream readings.
Why we don't list the exact part: A P0138 code tells you what's happening but not why. Replacing parts based on the code alone is the most common way drivers waste money. A proper diagnosis identifies the specific cause first.
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P0138 and Your MOT
A stored P0138 fault code will cause your car to fail its MOT. The engine management light will be illuminated during the test — an automatic failure under current MOT standards. Even if the light temporarily clears, the code remains in ECU memory and can be read by testers.
Get the fault diagnosed and repaired first, then have the code cleared and confirm the light stays off before booking your MOT.