P0143

What it means

P0143 means the engine control unit has detected a low voltage in the circuit of the oxygen sensor fitted at Bank 1, Sensor 3. This is the third sensor on the bank 1 side, normally fitted downstream of the catalytic converter, and not every vehicle has one. The control unit set the code because the sensor signal stayed below the expected range, or did not behave as expected, for longer than the test allows.

Symptoms

• Engine warning light on
• Usually no noticeable change in how the engine runs
• Possible slight rise in fuel consumption if other faults are also present
• Other oxygen sensor or catalyst efficiency codes stored alongside
• Possible failed emissions test

Possible causes

• Ageing or failed Bank 1 Sensor 3 oxygen sensor reading low or responding slowlylikely
• Damaged, chafed or corroded sensor wiring or connector, including a short to earthlikely
• Exhaust leak close to the sensor letting in extra air and skewing the readingpossible
• Sensor heater circuit fault, blown fuse or relay problem affecting the sensorpossible
• Contamination of the sensor by oil, coolant or fuel additivespossible
• Fault in the engine control unit or its sensor reference or earth supplyrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data, including engine temperature, load and speed when the code set
2. Confirm from the vehicle's wiring information which sensor is Bank 1 Sensor 3 on this model, as the layout varies with engine and exhaust
3. Visually inspect the sensor wiring and connector for heat damage, chafing, water ingress and corrosion
4. Check for exhaust leaks between the engine and the sensor, including joints and flexible sections
5. Check the relevant fuses and relays for the sensor heater, where fitted
6. Watch the live sensor voltage on a warm engine and see whether it moves with throttle changes or stays stuck low
7. Test the sensor circuit for continuity, shorts to earth and correct reference and heater supplies against the manufacturer's specification
8. Check the sensor heater resistance against the manufacturer's figure

How to fix

• Repair or replace damaged wiring and clean or replace corroded connectors
• Replace the Bank 1 Sensor 3 oxygen sensor if it fails the tests
• Repair any exhaust leak near the sensor
• Replace faulty fuses or relays for the sensor heater circuit
• Find and cure the source of any contamination, such as oil or coolant entering the exhaust, before fitting a new sensor
• Clear the code after repair and road test to confirm it does not return

Can I keep driving?

Nearby codes

• P0142GenericO2 Sensor Circuit Bank 1 Sensor 3
• P0144GenericO2 Sensor Circuit High Voltage Bank 1 Sensor 3