Oxygen (O2) sensor codes

How its codes differ

• Heater codes (P0135, P0141, P0031, P0037) are about the sensor's heater circuit, not its oxygen reading. P0135 and P0031 concern the bank 1 upstream sensor; P0141 and P0037 the bank 1 downstream sensor.
• P0137 (low voltage) and P0138 (high voltage) are about the bank 1 downstream sensor's reading. A reading stuck low or high can be the sensor, but a genuinely lean or rich exhaust or an exhaust leak near the sensor can cause the same reading.
• Converter codes (P0420, P0430) depend on the downstream sensors, so a downstream sensor fault should be fixed before a converter code is trusted.
• A slow or biased upstream sensor can make a fuel trim code (P0171, P0175) set when the mixture is actually fine; this is a lead to test rather than a common cause.

Codes

• P0135

  The heater in the upstream oxygen sensor on bank 1 isn't working as the computer expects

  Severity: Monitor

• P0141

  The heater in the downstream oxygen sensor on bank 1 isn't working as the computer expects

  Severity: Monitor

• P0137

  The downstream oxygen sensor on bank 1 is reporting a voltage that stays too low

  Severity: Monitor

• P0138

  The downstream oxygen sensor on bank 1 is reporting a voltage that stays too high

  Severity: Monitor

• P0031

  The heater control circuit for the upstream oxygen sensor on bank 1 is reading too low

  Severity: Monitor

• P0037

  The heater control circuit for the downstream oxygen sensor on bank 1 is reading too low

  Severity: Monitor

• P0420

  Catalytic converter efficiency below the computer's limit (Bank 1)

  Severity: Monitor

• P0430

  Catalytic converter efficiency below the computer's limit (Bank 2)

  Severity: Monitor

• P0171

  Engine computer has reached its limit adding fuel on bank 1: the mixture is lean

  Severity: Monitor

• P0175

  Engine computer has reached its limit removing fuel on bank 2: the mixture is rich

  Severity: Monitor

Signs it's failing

• Check engine light with an oxygen sensor code
• Poorer fuel economy
• A converter code that appears because the downstream sensor is faulty
• Sensor contamination (by coolant, silicone or heavy soot) that points to an engine problem as well

How to test it

1. Read the code and identify which sensor it names by bank and position (sensor 1 upstream, sensor 2 downstream). (Low risk)

2. For a heater code, inspect the sensor's wiring and connector for damage, melted insulation near the exhaust, or corrosion. (Use caution)

   Warning: Exhaust parts, the converter and nearby sensors get hot enough to cause serious burns and stay hot long after the engine is switched off. Work only on a cold engine, and never under a car held up only by a jack.

3. For a reading code, check fuel trims with a scan tool before replacing the sensor: a real lean or rich condition can make a good sensor read low or high. (Low risk)

4. Measuring heater resistance and sensor response against the vehicle's specification needs service information and a meter; have a shop do it if you are unsure. (Best left to a shop)

   Warning: Exhaust parts, the converter and nearby sensors get hot enough to cause serious burns and stay hot long after the engine is switched off. Work only on a cold engine, and never under a car held up only by a jack.

After the repair

• If the old sensor was contaminated, find and fix the source (coolant leak, oil burning, rich running) so the new sensor is not damaged.
• Clear codes only after the repair, then drive normally over several days with both city and highway driving so the readiness monitors can complete; clearing codes or disconnecting the battery resets them to not ready, which fails an emissions test.
• Confirm the new sensor responds normally on a scan tool once the engine is warm.