Code
P1151
SUBARU
P — Powertrain
Rear Oxygen Sensor Heater Circuit High Input
AI status
Completed
Completed
100%
Causes
- Short to battery voltage in the heater supply or sense wire
- Open or shorted wiring/connector (pinched, chafed, corroded)
- Failed oxygen sensor heater element or internal short
- Faulty heater driver or input circuit in the ECM/PCM
- Blown or incorrect fuse/relay in heater power feed
- Aftermarket repairs or incorrect sensor installed
Symptoms
- Check Engine Light (MIL) ON and stored P1151
- Possible failed emissions test
- Reduced fuel trim accuracy or drivability issues (may be subtle)
- Possible poor cold-start emissions until sensor reaches operating temperature
What to check
- Retrieve freeze frame and live data; note which bank/sensor is affected and heater control status
- Visually inspect rear O2 sensor wiring, connector, and nearby harness for damage, heat, or corrosion
- Check related fuses/relays for the heater circuit and replace if blown
- Measure heater circuit voltage with ignition ON (backprobe connector) and compare to expected
- Measure heater resistance at the sensor (with harness disconnected)
- Check for continuity to battery (+) and to chassis ground on appropriate pins to detect shorts
Signal parameters
- Typical downstream O2 sensor heater resistance: approx. 2–20 ohms (varies by sensor)
- Heater supply voltage (when energized): approx. battery voltage (11–14 V)
- Expected ECM sense voltage: near 0 V (if driver switches ground) or below threshold specified by manufacturer — a sustained high (> ~5–12 V depending on design) is abnormal
- Heater current draw when energized: typically 0.3–2 A (depends on sensor)
Diagnostic algorithm
- Verify the code and check for multiple related OBD codes (heater low/high, O2 signal faults, ECM faults).
- Perform a visual inspection of the rear O2 sensor connector and harness for damage, corrosion, heat exposure, or aftermarket splices.
- With ignition OFF, disconnect the rear O2 sensor. Measure heater resistance across the heater pins. Compare to typical range for that sensor (manufacturer spec preferred).
- Reconnect sensor. With ignition ON (engine OFF), backprobe the heater supply and control/sense wires. Measure voltage at the harness to check for an unexpected battery voltage on the sense/feedback line (indicates short to B+).
- Check for continuity between the heater power lead and battery positive, and between the heater control/sense lead and ground. Look for unintended shorts to battery or open circuits.
- Wiggle test the harness and connector while monitoring voltage to reproduce intermittent faults.
- If wiring and connector check OK but resistance indicates a shorted heater, replace the rear O2 sensor and retest.
- If new sensor still shows high input and wiring is verified good, suspect ECM/PCM heater driver or input circuit — consult wiring diagrams and consider ECU bench testing or replacement as a last resort.
- Clear codes and perform a road/test cycle to confirm repair. Re-scan for pending or permanent codes.
Likely causes
- Wiring shorted to 12V (most common for a 'high input' reading)
- Corroded or damaged sensor connector causing incorrect voltage at the ECM
- Failed downstream O2 sensor heater with internal short
- Faulty ECM heater control/sense circuit (less common)
Fault status
Status
P1151 — Rear Oxygen Sensor Heater Circuit High Input: ECM detected heater circuit voltage above expected threshold for the downstream O2 sensor; inspect wiring, connector, sensor, fuses, and ECM heater driver.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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