P0166 — No signal from rear oxygen sensor on bank 2
The engine control unit is not receiving any signal from the rear oxygen sensor on the second bank of cylinders. This sensor monitors the efficiency of the catalytic converter and helps manage emissions. The code appears when the sensor circuit shows no activity, possibly due to wiring issues, a failed sensor, or connector problems.
Common causes
The cause is almost always one of a few things. Here they are — P0166 rarely comes from anything outside this list:
- Faulty or dead oxygen sensor (bank 2 sensor 3)
- Damaged or corroded wiring/connectors to the sensor
- Exhaust leaks near the sensor
- Failed catalytic converter affecting sensor readings
Where to start
In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:
- Inspect and repair wiring and connectors
- Check for exhaust leaks and fix if present
- Test sensor output with diagnostic tools
- Replace oxygen sensor if faulty
- Inspect catalytic converter if sensor replacement doesn’t help
That is what holds for P0166 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
- AUDI: Checking Lambda probes and Lambda control after catalytic converter - vehicles with engine code letters "“ARE”", "“AZA”" and "“BES”"
- VW: Removing and installing Lambda probe 3 after catalytic converter G287 and Lambda probe 4 after catalytic converter G288
- 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
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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