P1143 — Heated oxygen sensor (HO2S) 2, bank 1 - signal too high

Diagnostic algorithm

1. Verify code and capture freeze-frame data; note operating conditions when fault set.
2. With a scan tool, observe live data: B1S1 and B1S2 voltages, heater status, short-term/long-term fuel trims, and fuel pressure if available.
3. If B1S2 reads abnormally high, backprobe the sensor connector and confirm the voltage with a multimeter. Check for constant high voltage or intermittent spikes.
4. Inspect connector and wiring for damage, pin corrosion or evidence of short to battery. Wiggle harness while monitoring signal for intermittent faults.
5. Check continuity between sensor signal pin and ECU; check for short to 12V and to ground.
6. Measure heater resistance and verify heater supply voltage and ground when heater is commanded on. Repair open/shorts as found.
7. If wiring and heater are good, evaluate fuel system: check fuel pressure, injector leaks, and fuel trims for evidence of a rich condition that could cause a high downstream voltage.
8. Compare upstream and downstream O2 behavior. If upstream indicates proper switching and downstream remains high, consider sensor replacement or catalytic converter contamination/failure.
9. Replace only after confirming wiring and fuel/system conditions are not the root cause. Clear codes and perform road test to confirm the code does not return.
10. If code persists after sensor replacement, inspect ECU outputs and grounds; consider professional-level electrical diagnosis.