An O2 (oxygen) sensor measures how much oxygen is left in the exhaust so the engine computer can keep the air-fuel mixture correct. Most modern vehicles have at least two: an upstream sensor before the catalytic converter that controls fueling, and a downstream sensor after it that monitors the converter. V6 and V8 engines usually have four, two per bank. A failing O2 sensor causes a check engine light, worse fuel economy and sometimes rough running.
Upstream vs downstream O2 sensor
| Sensor | Location | Job |
|---|---|---|
| Sensor 1 (upstream) | In the exhaust manifold or downpipe, before the converter | Fuel control; the computer adjusts fuel trims from its signal |
| Sensor 2 (downstream) | After the catalytic converter | Monitors converter operation; little effect on fueling |
Bank 1 is the side of the engine containing cylinder 1 and bank 2 is the opposite side, so “bank 1 sensor 2” means the downstream sensor on the cylinder 1 side. Many newer engines use a wide-band air-fuel ratio sensor upstream, which reports the mixture more precisely than a traditional switching sensor.
Bad O2 sensor symptoms
- Check engine light with codes such as P0135, P0141, P0133 or P0131
- Lower fuel economy, because the computer falls back to default fueling
- Rough idle or hesitation on some vehicles; see rough idle
- Black smoke or a fuel smell if the engine runs rich
Common O2 sensor codes
| Code | Meaning |
|---|---|
| P0131 | Low voltage, bank 1 sensor 1 |
| P0133 | Slow response, bank 1 sensor 1 |
| P0134 | No activity, bank 1 sensor 1 |
| P0135 | Heater circuit, bank 1 sensor 1 |
| P0137 / P0138 | Low / high voltage, bank 1 sensor 2 |
| P0141 | Heater circuit, bank 1 sensor 2 |
How long do O2 sensors last?
Modern heated sensors often last somewhere between 60,000 and 100,000 miles. They fail sooner when the engine burns oil or coolant, which coats the sensing element; if new sensors keep failing, check for blue or white smoke and a head gasket leak.
Before you replace an O2 sensor
- A lean or rich code (P0171, P0172) usually means the sensor is reporting a real problem, not that it has failed.
- Exhaust leaks ahead of a sensor pull in air and make it read lean.
- Heater codes are often a blown fuse or damaged wiring along the exhaust.
Replacing an O2 sensor
- Unplug the connector and use an O2 sensor socket, which has a slot for the wire.
- Penetrating oil and a slightly warm (not hot) exhaust make a seized sensor easier to remove.
- New sensors usually come with anti-seize on the threads; keep it off the sensor tip.
- Route the new wiring away from the exhaust, clear the codes and confirm the readings on a scan tool.
If an engine is contaminating sensors because it is worn out, compare a replacement on our engines page.
More guides: fuel trim explained · exhaust leak symptoms.
Trouble codes: P2195 O2 Sensor Stuck Lean B1S1 · P2196 O2 Sensor Stuck Rich B1S1 · P0030 O2 Heater Circuit B1S1 · P0132 O2 Sensor High Voltage B1S1 · P0136 O2 Sensor Circuit B1S2 · P0151 O2 Sensor Low Voltage B2S1 · P0154 O2 Sensor No Activity B2S1 · P0155 O2 Heater Circuit B2S1 · P0157 O2 Sensor Low Voltage B2S2 · P0158 O2 Sensor High Voltage B2S2 · P0161 O2 Heater Circuit B2S2 · P0130 O2 Sensor Circuit B1S1 · P0139 O2 Sensor Slow Response B1S2 · P0140 O2 Sensor No Activity B1S2 · P0153 O2 Sensor Slow Response B2S1 · P0156 O2 Sensor Circuit B2S2 · P0159 O2 Sensor Slow Response B2S2 · P2270 O2 Sensor Stuck Lean B1S2 · P2271 O2 Sensor Stuck Rich B1S2 · P2272 O2 Sensor Stuck Lean B2S2 · P2273 O2 Sensor Stuck Rich B2S2 · P0031 O2 Heater Circuit Low B1S1 · P0032 O2 Heater Circuit High B1S1 · P0037 O2 Heater Circuit Low B1S2 · P0038 O2 Heater Circuit High B1S2 · P0051 O2 Heater Circuit Low B2S1 · P0052 O2 Heater Circuit High B2S1 · P0057 O2 Heater Circuit Low B2S2 · P0058 O2 Heater Circuit High B2S2 · all OBD2 codes.
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