Code P1158 MERCURY – Causes, Symptoms, and Repairs
1. Meaning of Code P1158 MERCURY
The fault code P1158 is related to the Heated Oxygen Sensor 2 (HO2S2) located downstream of the three-way catalyst. This sensor monitors the oxygen level in the exhaust gases and allows the Powertrain Control Module (PCM) to adjust the air-fuel ratio to optimize combustion.
The sensor generates a voltage between 0.1V and 0.9V:
- Voltage < 0.45V → lean mixture (too much oxygen)
- Voltage > 0.45V → rich mixture (too much fuel)
When this sensor does not function properly, the PCM may not be able to correctly adjust fuel injection, leading to the appearance of code P1158.
2. Possible Symptoms
- Engine light on (Check Engine)
- Fuel consumption issues
- Irregular idle or loss of power
3. Possible Causes of Code P1158 MERCURY
Several factors can cause this error code:
✅ Exhaust gas leaks
✅ Faulty Heated Oxygen Sensor 2 (HO2S2) (Bank 2, downstream)
✅ Open or short circuit in HO2S2 sensor wiring
✅ Poor electrical connection of HO2S2 sensor
✅ Inappropriate fuel pressure
✅ Faulty fuel injectors
4. How to Repair Code P1158 MERCURY?
Here are the recommended steps to diagnose and resolve this issue:
🔍 Step 1: Checking Connections and Electrical Wiring
✔️ Visually inspect the oxygen sensor wiring harness and connectors.
✔️ Look for broken, bent, pinched, or corroded wires.
🔍 Step 2: Checking the HO2S2 Oxygen Sensor
✔️ Test the sensor with a multimeter:
- Check the output voltage (should be between 0.1V and 0.9V).
- Replace the sensor if it does not respond correctly to air-fuel mixture variations.
🔍 Step 3: Checking for Exhaust Leaks
✔️ Inspect the exhaust manifold and catalyst for possible leaks.
🔍 Step 4: Checking Fuel Pressure and Injectors
✔️ Check fuel pressure using a pressure gauge.
✔️ Test fuel injectors and replace any that are clogged or faulty.
5. Conclusion
Code P1158 on a MERCURY vehicle is often related to a faulty oxygen sensor or an electrical issue. A thorough diagnosis is necessary to avoid unnecessary part replacement. By following these steps, you can effectively identify and fix the problem.
🚗 Tip: If you are unsure about the diagnosis, use an OBD2 diagnostic tool or consult a professional.