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Symptoms of a Worn Cam-Angle Sensor
When the Subaru Impreza's engine check lamp comes on, one overlooked cause is a worn cam-angle sensor. As the sensor degrades, the camshaft's position is no longer reported accurately to the engine control unit (ECU), throwing off ignition and fuel timing. The result is reduced power and a lit warning light. Most owners first suspect a faulty O2 sensor or a loose gas cap, but a worn cam-angle sensor is frequently the real culprit.
- •Reads cam position accurately
- •Ignition & fuel timing optimal
- •Stable power output
- •Check lamp stays off
- •Position signal gets inaccurate
- •Timing drifts off
- •Power drop & hesitation
- •Check lamp turns on
Typical signs include the warning light plus hesitation under acceleration, rough idling, and a drop in power. Replacing the sensor restores smooth performance and clears the diagnostic trouble codes (DTCs).
Why the Cam-Angle Sensor Wears Out
The cam-angle sensor reads the rotation angle of the camshaft and sends that information to the ECU. Over years of use, its internal parts and contacts degrade, the position signal becomes noisy, and the ECU can no longer calculate correct timing. That is the starting point for power loss and the warning light.
Telling It Apart From the O2 Sensor
While the cam-angle sensor is the main focus, the same check lamp can also involve a degraded O2 sensor. The O2 sensor monitors oxygen in the exhaust and tells the ECU how much fuel to inject. Because their roles differ, don't judge by symptoms alone; use trouble codes to separate the two. When both are worn, some owners replace them together within the same service window.
- •Detects camshaft position
- •Controls ignition & fuel timing
- •Wear scrambles the position signal
- •Monitors oxygen in exhaust
- •Trims the air-fuel mixture
- •Aging hurts economy & performance
For the O2 sensor's own effects, see our guide on how the Impreza's oxygen sensor drives economy and performance.
Where the Sensor Sits and What to Inspect
The cam-angle sensor is mounted on the timing cover at the rear of the engine block, near the back of the cylinder head. On most Impreza models it sits toward the rear of the engine and is reachable once the engine cover is removed. Look for a small, round sensor with a wiring harness attached.
During inspection, check not only the sensor body but also the wiring harness and terminals for cracks, looseness, or corrosion. Checking both visually and via trouble codes makes it easier to tell wear apart from a wiring fault.
Deciding On and Doing the Replacement
Once wear is confirmed, replacement is the fix. You'll want a 10-mm or 12-mm socket, a torque wrench, a new sensor matching the OEM part number, a diagnostic scan tool, and basic hand tools. Disconnect the battery before you start to avoid electrical shocks. Tighten to the manufacturer's torque spec, usually around 10-12 Nm.
DIY keeps labor costs down, but if you're unsure about torque control or handling the wiring, having a repair shop do it is a sound choice.
- •Needs 10/12mm sockets and tools
- •Saves on labor cost
- •Watch your torque
- •Reliable diagnosis and work
- •May come with a warranty
- •Adds a labor charge
For the detailed removal and installation, also see our step-by-step DIY cam-angle sensor replacement guide.
Key Takeaways
The Impreza's engine check lamp can be driven mainly by a worn cam-angle sensor. Understand the symptoms and causes, and use trouble codes to tell it apart from the O2 sensor, and you can avoid needless part swaps. Once wear is confirmed, replace the sensor at around 10-12 Nm, clear the DTCs, and verify with a test drive.
- •Blame only the O2 sensor
- •Assume a loose gas cap
- •Give up expecting a costly repair
- •Cam-angle wear is often the main cause
- •Replacement clears the warning
- •DIY can keep costs down