P0138 OBD-II code — what it means and what to check
ℹ Guidance only — not a substitute for inspectionEnter the trouble codes from your scanner to see what each one means, how urgent it is, and which checks to do first.
Your codes, interpreted
What this means
⚠ Book a service. The codes you entered describe faults that get worse, or more expensive, if left.
What to doBook a workshop inspection. Normal driving is usual with these faults, but they should not be left indefinitely.
Your codes
- P0138 — Emissions control (service soon). The oxygen sensor after the catalytic converter on bank 1 is holding a high reading instead of cycling. This sensor watches the converter rather than trimming the fuel, so the fault is usually the sensor or its wiring rather than the mixture.
Checks to do first, in order
- Inspect the sensor's connector and wiring for corrosion, melted insulation or a chafed lead — this is the most common cause.
- Check for an exhaust leak ahead of the sensor, which lets air reach it and distorts what it reports.
- Have the sensor's live reading watched while the engine warms: a healthy one swings, a failed one sits still.
- Look for a leaking injector or another cause of a genuinely rich mixture if the sensor itself checks out.
Codes this tool cannot interpret
How this result was reached
- Interpretations cover standardised codes only, and describe what a code means rather than what is wrong with your specific vehicle.
- A stored code identifies the circuit or system reporting a fault. It does not identify the failed part — a sensor commonly reports a problem it did not cause.
- Reviewed by Project Owner on 2026-08-26.
- This tool covers a reviewed subset of standardised codes. A code it does not recognise is reported as undocumented rather than guessed at.