What does it mean?
P1172 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Rear Air/Fuel Ratio (A/F) Sensor (Bank 1, Sensor 1) Circuit Out of Range High .
Symptoms
- Check engine light on
- Fuel control on that bank is disabled, so mixture is held on a default map
- Rough idle and hesitation, sometimes a slight surge at steady speed
- Increased fuel consumption and exhaust smell
- Fuel trims on one bank clearly different from the other
- Emissions test failure
Likely causes
- Short to voltage in the sensor signal wire, often where the harness runs behind the engine near heat and moving parts
- Open sensor ground, which lets the signal float high
- Internal failure of the air/fuel sensor element
- Water or corrosion in the sensor connector, common on rear-bank sensors that sit low and are hard to see
- Terminal damaged or pushed back during previous exhaust or spark plug work
- Rarely, a control module input circuit fault
How to fix it
- Read live data and compare the two banks; note whether the suspect value is pinned at its maximum
- Unplug the sensor and check whether the reading changes, which separates a wiring short from a sensor fault
- Measure the signal wire for a short to battery voltage with both ends disconnected
- Check sensor ground continuity to the module and its resistance to chassis ground
- Inspect the connector for water ingress, green corrosion and pin tension; dry and reseal if needed
- Repair chafed harness sections and reroute away from the exhaust
- Replace the air/fuel sensor with the correct part if the circuit tests good, then clear the code and verify both banks track together