Emissions

P0143 — O2 Sensor Circuit Low Voltage (Bank 1, Sensor 3)

Severity: low Safe to drive

What P0143 means

The ECU monitors the third oxygen sensor on the Bank 1 side of the engine. This is usually a sensor located after the catalytic converter, or after an additional converter on systems with two. Its signal normally stays within a working range, but with P0143 the voltage remains too low. The cause can be the sensor itself, its wiring, or an exhaust leak near it.

Symptoms

  • Check Engine light is on
  • Slightly higher fuel consumption
  • Possible rough or unstable idle
  • Possible hesitation when accelerating
  • Higher chance of failing an emissions test

Possible causes

  1. Failed Bank 1 Sensor 3 oxygen sensor (most likely)
  2. Damaged, corroded or loose sensor wiring or connector
  3. Signal wire shorted to ground
  4. Exhaust leak near the sensor
  5. Poor ground or reference voltage problem
  6. Faulty ECU (rare)

How to diagnose

  1. Scan for codes and freeze frame data, and note any other sensor or fuel trim codes
  2. Confirm which sensor your vehicle calls Bank 1 Sensor 3
  3. Inspect the wiring harness and connector for melting, chafing, corrosion or moisture
  4. Check the exhaust for leaks near the sensor and converter
  5. Watch live sensor voltage and see whether it responds to engine changes
  6. Review fuel trims, since large deviations point to a mixture problem
  7. Test the signal wire for a short to ground and check the circuit's power and ground
  8. If the wiring is good, replace the sensor and recheck

Common fixes

  • Replace the Bank 1 Sensor 3 oxygen sensor
  • Repair or replace damaged wiring and connectors
  • Fix the exhaust leak ahead of the sensor
  • Restore the ground or reference voltage circuit
  • Repair or replace the ECU only after ruling out everything else

Questions and answers

What does P0143 mean?

The ECU detected low voltage in the Bank 1 Sensor 3 oxygen sensor circuit. It is most often the sensor after the catalytic converter or its wiring.

Can I drive with code P0143?

Usually yes, but don't put off diagnosis. It can raise fuel use and worsen emissions over time.

Will replacing the sensor fix it?

Not always. Check the wiring, connector and exhaust leaks first, or a new sensor may set the same code.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.