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DTC codes / Subaru codes / P1150

P1150 · SubaruFront Oxygen Sensor Heater Circuit High Input

P1150 on a Subaru means the engine control module (ECM) has detected an abnormally high voltage on the heater circuit of the front air/fuel ratio sensor, which is the sensor mounted ahead of the catalytic converter. The ECM expects the heater circuit to draw current and pull the monitoring voltage low when energized, so a high reading tells the computer the circuit is open, has excessive resistance, or is shorted to voltage.

SystemPowertrain
Code typeSubaru specific
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms.
  • A slight delay in the air/fuel sensor reaching operating temperature, which can cause a brief rich-running condition after a cold start.
  • Occasional rough idle or hesitation during the first minute or two of driving after a cold start.
  • Slightly reduced fuel economy if the sensor heater stays inactive and the ECM relies on default fueling strategies.
  • In rare cases, a stored pending code that clears itself after the engine warms up if the fault is intermittent.
  • No drivability complaint at all in many cases, because the heater only affects cold-start emissions and sensor readiness.

Common causes, most likely first

  1. A failed front air/fuel ratio sensor heater element, which is the most common cause on higher-mileage Subaru engines.
  2. Corroded, pushed-back, or damaged terminals at the front air/fuel sensor connector or the ECM connector.
  3. A broken or chafed wire in the heater circuit harness, often near the exhaust manifold where heat and vibration take a toll.
  4. A short to voltage on the heater control wire, which can be caused by harness contact with a hot exhaust component or another powered circuit.
  5. An internal fault in the ECM driver for the heater circuit, though this is far less common than sensor or wiring failures.
  6. A poor ground connection for the heater circuit at the engine or chassis grounding point, which raises the measured voltage on the control side.
  7. Aftermarket or incorrect sensor replacement that does not match the Subaru heater resistance specification.

How to diagnose and fix it

  1. Connect a scan tool and confirm P1150 is current or pending, then check freeze-frame data for engine coolant temperature and sensor heater status to understand when the fault set.
  2. Inspect the front air/fuel sensor connector and the harness routing for corrosion, pushed terminals, chafing, or contact with hot exhaust parts, repairing any damage found.
  3. Measure the resistance of the heater element at the sensor connector with a multimeter and compare it to the Subaru specification for the engine in the vehicle.
  4. With the key on and the sensor disconnected, check for reference voltage on the heater control wire and verify the ground side has less than one ohm to battery negative.
  5. Perform a voltage drop test on the heater circuit while it is commanded on, looking for excessive resistance in the wiring or grounds that would drive the ECM voltage high.
  6. If the wiring and grounds test good but the heater resistance is out of specification or open, replace the front air/fuel ratio sensor with the correct Subaru-specified part.
  7. Clear the codes, run the vehicle through a cold-start and warm-up cycle, and verify the heater monitor completes without P1150 returning.

What the heater circuit voltage really tells you

  • A high voltage reading on the heater circuit does not automatically mean the sensor is bad, because an open heater wire in the harness or a corroded ground can produce the exact same code.
  • Subaru front air/fuel ratio sensors are not the same as conventional oxygen sensors, and the heater resistance specification differs between engine families, so checking the correct service information for the vehicle is essential before condemning the sensor.
  • Intermittent P1150 codes that appear only on cold damp mornings often trace to a chafed heater wire near the exhaust manifold that makes contact only when the harness is cold and stiff.
  • A failed heater can cause the ECM to stay in open-loop fueling longer than normal, which may set companion mixture codes even though the driver notices nothing beyond the warning light.
  • Technicians sometimes replace the sensor only to have P1150 return because the ECM connector terminals were spread and making poor contact, so a terminal drag test on both ends of the circuit is worth the time.

Can I keep driving with P1150?

You can usually keep driving with P1150 because the heater fault mainly affects cold-start emissions and sensor readiness, but avoid long periods of short-trip driving that never let the engine fully warm up. If the check engine light begins flashing or you notice rough running, stop driving and have the vehicle diagnosed promptly.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Subaru vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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