Code
P1125
PORSCHE
P — Powertrain
Oxygen sensing, area 1, cylinders 4-6
Views:
UK: 4
EN: 8
RU: 6
AI status
Completed
Completed
100%
Causes
- Faulty oxygen (lambda) sensor in area 1 (cylinders 4–6)
- Damaged wiring or poor connector/ground in the sensor circuit
- Open or short in heater circuit (if sensor is heated)
- Exhaust leak upstream of the sensor
- Sensor contaminated by oil, coolant or fuel additives
- Fuel delivery or injection problem causing incorrect AFR
Symptoms
- Check Engine Light / MIL illuminated
- Increased fuel consumption or poor fuel economy
- Rough idle or misfire-like symptoms from cylinders 4–6
- Reduced engine performance or hesitation
- Failed emissions test / elevated tailpipe emissions
- Possible drivability intermittence (stalling, surging)
What to check
- Read and record freeze-frame / live data with a capable scan tool (O2 voltage/current, lambda, fuel trims)
- Inspect sensor connector and wiring for damage, corrosion, loose pins, or rodent chew
- Check heater circuit power, ground and fuse/relay operation (if applicable)
- Measure sensor signal voltage/current while engine at operating temperature and in closed-loop
- Compare upstream vs downstream sensor behavior (if equipped with multiple sensors)
- Perform smoke or visual test for exhaust leaks upstream of the sensor
Signal parameters
- Narrowband zirconia sensor: switching voltage ~0.1–0.9 V (oscillates when closed-loop)
- Heated sensor heater resistance (typical range): approx. 2–20 ohms (manufacturer-specific)
- Wideband/UEGO sensors: output often 0–5 V or a current signal 0–20 mA depending on design — consult factory data
- Expected switching frequency: up to ~1 Hz or faster in normal closed-loop operation
- Response time: sensor should respond to AFR changes within ~100–1000 ms (manufacturer-specific)
- Open/short thresholds: signal stuck near 0 V or near reference voltage/rail indicates fault
Diagnostic algorithm
- Retrieve full code list, freeze-frame data and live O2 sensor data with scan tool; record fuel trims and engine conditions when code set.
- Visually inspect the sensor, connector and wiring harness for damage, corrosion, melted insulation or poor connections. Repair as needed.
- Verify battery/charging voltage and inspect fuse/relay for the oxygen sensor heater circuit (if sensor is heated).
- With connectors connected, back-probe the sensor signal and heater terminals. Confirm heater supply and ground, and measure heater resistance with the sensor disconnected if required.
- Start engine and observe live sensor signal: a narrowband should switch between low (~0.1 V) and high (~0.7–0.9 V) once warmed; a wideband should show a plausible lambda/current/voltage trace. Compare to factory data.
- If the signal is stuck or absent, check continuity and for short to power/ground between sensor and ECU; repair wiring faults.
- Check for exhaust leaks upstream of the sensor and repair if present, then retest.
- Inspect fuel system (pressure, fuel injectors for cylinders 4–6) and ignition for issues that could bias O2 readings; correct any problems.
- If wiring, power, grounds and engine mechanicals are good but sensor signal remains out of range, replace the oxygen sensor (use OEM or specified part).
- After repair, clear codes and perform a drive cycle or readiness test to confirm the fault does not return. If it does, consider ECU/PCM diagnosis or replacement as a last resort.
Likely causes
- Failed oxygen sensor (age, contamination, internal failure)
- Broken/frayed wiring harness or corroded connector at sensor
- Blown fuse or bad relay for sensor heater circuit
- Exhaust manifold or gasket leak near cylinders 4–6
- High/low fuel pressure or leaking injector(s) on cylinders 4–6
- Short to voltage or ground on the signal wire
Fault status
Status
Oxygen-sensor circuit for area 1 (cylinders 4–6) is reporting an invalid or out-of-range signal or is not switching as expected. Inspection of sensor, wiring, heater circuit and related systems is required.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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