P1143: Rear O2 Sensor Stuck Rich Bank 1
The P1143 code indicates that the Engine Control Module (ECM) has detected the rear heated oxygen sensor (HO2S) on Bank 1 (Sensor 2) is not switching between rich and lean, staying at a consistently high voltage (rich condition). This sensor monitors catalytic converter efficiency; a stuck-rich signal prevents the ECM from verifying the catalyst's oxygen storage capacity and adjusting long-term fuel trim.
01. Most Common Causes
- Contaminated, aged, or internally damaged HO2S Bank 1 Sensor 2
- Exhaust leak before or after the sensor (false air entry)
- Leaking fuel injector or excessive fuel pressure (actual rich condition)
- Sensor wiring or connector shorted to voltage (fixed high signal)
- Faulty ECM (rare)
02. Symptoms You Will Notice
- Check Engine Light on
- Failed emissions test due to catalyst efficiency monitor not ready
- Possible poor fuel economy or rough idle if rich condition is real
03. Step-by-Step Diagnosis
Scan for freeze frame data and verify P1143 is active
Visually inspect HO2S wiring and connector for damage or corrosion
Check for exhaust leaks near the sensor using a smoke machine or soapy water
Monitor O2 sensor voltage with a scan tool; if stuck above 0.8V, test sensor response by inducing lean/rich conditions
Test fuel pressure and injector leak-down to rule out mechanical rich condition
1. Frequently Asked Questions
Is it safe to drive?▼
Yes, but the vehicle may fail an emissions test and fuel economy could suffer. Have it diagnosed soon to prevent catalytic converter damage.
How much does it cost to fix?▼
Typically $150–$400 for sensor replacement, plus diagnostic time. Costs can be higher if exhaust leaks or fuel system repairs are needed.