P0138 — Oxygen sensor signal voltage is too high
The oxygen sensor after the catalytic converter is sending a voltage signal that is higher than expected. This usually means the sensor is detecting too much fuel or a problem with the sensor’s wiring or heater circuit. It can cause poor fuel economy and increased emissions but usually won’t cause immediate engine damage.
Common causes
The cause is almost always one of a few things. Here they are — P0138 rarely comes from anything outside this list:
- Faulty oxygen sensor (bank 1 sensor 2)
- Damaged or corroded sensor wiring/connectors
- Exhaust leaks near the sensor
- Contaminated or failing catalytic converter
Where to start
In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:
- Inspect and repair sensor wiring and connectors
- Check for and fix exhaust leaks near sensor
- Clean or replace the oxygen sensor
- Test catalytic converter function if problem persists
That is what holds for P0138 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.
Why TorqFix for your car?
Above is what the code means and why it appears. TorqFix goes further — it gives you specific procedures, torque values, wiring diagrams and pinouts for your exact engine, not generic steps for every car. Real workshop data (127,000+ diagrams, 10,000+ procedures), interpreted by AI in your language.
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Code breakdown
🔧 Related repair procedures
- VW: Removing and installing catalytic converter
- AUDI: Removing and installing front exhaust pipe (right-side) with catalytic converter - vehicles with multitronic gearbox
- AUDI: Checking Lambda probe and Lambda control before catalytic converter
- AUDI: Removing and installing front exhaust pipe (left-side) with starter catalytic converter and main catalytic converter (vehicles with automatic gearbox)
- AUDI: Removing and installing starter catalytic converter
- AUDI: Removing and installing front exhaust pipe (left-side) with starter catalytic converter and main catalytic converter (vehicles with manual gearbox)
- AUDI: Removing and installing front exhaust pipe (right-side) with starter catalytic converter and main catalytic converter (vehicles with automatic gearbox)
- VW: Removing and installing catalytic converters
- VW: Removing and installing starter catalytic converter
- VW: Checking Lambda probe and Lambda control before catalytic converter
Who makes these pages, and how
TorqFix was built by a mechanic — after a 2,700 km drive with five roadside repairs that no diagnostic tool could explain.
The figures on this page come from manufacturer workshop documentation and are shown as they are. The page is assembled automatically from that data — it is not written by an AI. The AI works in the chat, where it answers for your specific car. It writes nothing here.
The same code can mean different things on different makes. Match the meaning to your make before acting on it.
Seen a figure that does not match your manual? Write to us — we correct it and note when. support@torqfix.com
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