P0143
O2 Sensor Circuit Low Voltage (Bank 1, Sensor 3)
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The P0143 code means the PCM has detected that the voltage signal from the third oxygen sensor on Bank 1 is staying below the expected minimum threshold. Normally, an O2 sensor's voltage should oscillate between approximately 0.1V and 0.9V as it detects rich and lean exhaust conditions. When the signal stays stuck near the low end, the PCM flags this code.
Sensor 3 on Bank 1 is positioned after a secondary catalytic converter and is primarily used to verify emissions system performance. Because of this monitoring-only role, a low-voltage reading from this sensor generally doesn't cause noticeable drivability issues for the driver. The main impact is a persistent check engine light and emissions test failure.
The most common cause is a worn-out sensor that needs replacement. However, before swapping the sensor, check for exhaust leaks near its location and inspect the wiring harness for damage or shorts. If the vehicle has been exposed to coolant leaks or had silicone sealant used in engine repairs, these substances can contaminate the sensor and cause it to read low. This is a moderately easy DIY repair, though the sensor's location under the vehicle may require a lift or jack stands.
Severity
Symptoms
- •Check engine light is on
- •Vehicle fails emissions testing
- •Possible very slight decrease in fuel economy
- •No major change in driving feel
- •Exhaust odor may be slightly different
Likely Causes
The sensing element has degraded and is stuck producing a low voltage output, indicating a constant lean reading regardless of actual exhaust conditions.
An exhaust leak upstream of Sensor 3 allows ambient air to enter the exhaust stream, producing a genuinely lean reading at the sensor that the PCM interprets as a stuck-low condition.
A wire in the sensor's signal circuit that is shorted to ground or has an open connection will result in a persistently low voltage at the PCM.
Exposure to silicone sealants, coolant (from a head gasket leak), or excessive oil can coat the sensor element and cause it to read abnormally low.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the O2 sensor (Bank 1, Sensor 3)
- Repair exhaust leaks near the sensor location
- Repair or replace damaged signal wiring and connectors
- Address any coolant or oil leaks that may be contaminating the sensor