Code
P0143
GWM
P — Powertrain
- Low oxygen circuit voltage (bank 1, sensor 3)
AI status
Completed
Completed
100%
Causes
- Faulty/contaminated downstream O2 sensor (Bank 1, Sensor 3)
- Short to ground in the O2 sensor signal circuit
- Open circuit or high resistance in signal wiring or connector
- Connector corrosion, bent pins, or poor contact
- Exhaust leak or damage upstream of the sensor affecting readings
- Fuel trim or combustion conditions causing abnormally low sensor voltage
Symptoms
- Check Engine Light (MIL) illuminated
- Failed emissions test or reduced catalyst monitoring readiness
- Possible reduced fuel economy or drivability issues if long-term
- No obvious engine misfire (downstream sensors typically don't control fuel trim)
What to check
- Read and record freeze-frame and live data for Bank 1 Sensor 3 and compare to Bank 1 Sensor 2 (upstream).
- Inspect sensor connector and wiring for damage, chafing, corrosion, or heat exposure.
- Check for exhaust leaks near the sensor or upstream (manifold/gasket/downpipe).
- Backprobe sensor signal with scan tool or multimeter to confirm voltage behavior during different operating conditions.
- Measure heater circuit resistance and supply voltage (if equipped with heated O2 sensor).
- Wiggle test wiring while monitoring signal to reproduce fault.
Signal parameters
- Typical narrowband O2 sensor voltage: ~0.1–0.9 V (switching for upstream sensors); downstream sensor usually steadier but should be >~0.1 V and responsive to changes.
- DTC triggered when sensor signal falls below manufacturer threshold for a set time (varies by ECU).
- Heater element resistance (heated sensors): commonly between ~2–20 ohms (vehicle-specific).
- Heater supply voltage: battery voltage when ignition on; control via ECU (check with live data).
- Expected behavior: downstream sensor should not be stuck at a low constant voltage; should reflect catalytic and exhaust conditions.
Diagnostic algorithm
- Verify fault: retrieve freeze-frame data and confirm P0143 is current/persistent. Check related codes (heater, upstream O2).
- Visual inspection: with engine off and cooled, inspect Bank 1 Sensor 3 connector and wiring for damage, corrosion, or heat damage. Repair visible damage.
- Check for exhaust leaks: inspect exhaust gasket, pipes and joints upstream of the sensor location; repair any leaks.
- Monitor live data: start engine and observe Bank 1 Sensor 3 voltage with a scan tool. Compare to Bank 1 Sensor 2. A persistently low voltage on Sensor 3 suggests a circuit or sensor issue.
- Backprobe signal: with digital multimeter (DC volts) or oscilloscope, backprobe the signal wire at the connector while engine running. Confirm voltage is below expected and whether it changes with throttle/air/fuel conditions.
- Wiggle test: gently move wiring/harness while observing the signal for intermittent changes. If voltage changes, repair/replace harness or connector.
- Check heater circuit: with ignition off, measure heater element resistance between heater pins. Then with ignition on, verify heater supply voltage. Replace sensor if heater open/short or no supply when commanded.
- Check for short to ground/power: disconnect sensor and measure continuity from signal wire to ground/power. Repair short/open as needed.
- If wiring and connectors test good and heater/supply are correct, replace the Bank 1 Sensor 3 with a correct part and clear codes.
- Re-test: clear codes, road test, and verify no return of P0143 and that monitor readiness is achieved.
Likely causes
- Damaged or failed Bank 1 Sensor 3 (contaminated or electrically failed)
- Signal wire shorted to ground or chafed against chassis/exhaust
- Corroded/loose connector at the sensor or harness
- Exhaust leak between upstream and downstream sensors or near sensor location
- Heater circuit problems (if heater-related diagnostics are present)
Fault status
Status
ECM detected a low voltage condition on the Bank 1, Sensor 3 oxygen sensor circuit. This indicates the sensor signal is below the expected threshold for a sustained period, suggesting a sensor, wiring/connector fault, exhaust leak, or related fault. Further testing is required before repair.
Repair difficulty: Medium
Diagnostic time: 0.5-2 hours
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