P1289

P1289 is manufacturer specific and has no single SAE or ISO definition applicable to every vehicle. The GM service definition commonly associated with this code is Cylinder Head Temperature Sensor Circuit High Voltage. Confirm the exact definition in GM service information using the vehicle VIN and calibration.

Definition source: Historical source not established

Definition verification: Not independently verified for display.

Time: About 45 minutes to 1.5 hours for diagnosis and a straightforward sensor or connector repair; access and wiring repairs can add time. | Diagnostic difficulty: Medium

Medium Severity

The cylinder head temperature sensor reports engine metal temperature to the ECM. On applications using this sensor, the ECM expects a voltage that changes with temperature; an abnormally high circuit voltage can indicate an open circuit, poor connection, or sensor fault, so the ECM may receive an implausibly cold reading and make incorrect temperature based decisions.

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⚠️ Before You Begin (Safety First)

A bad temperature signal can keep the ECM from recognizing actual cylinder head heat promptly. If the engine is overheating, continuing to drive can cause severe engine damage; stop safely if the temperature warning appears, steam is present, or power drops. Do not open a hot cooling system.

Common Symptoms

The malfunction indicator lamp may turn on because the ECM detects a voltage outside its expected range during its circuit checks.
A cold start may run poorly if the ECM interprets the high voltage as an unrealistically cold engine and adjusts fueling or other temperature based controls.
Temperature readings may be implausible or fixed because an open or high resistance circuit prevents the sensor signal from tracking engine heat.
Overheating may not be identified promptly if the faulty signal makes the ECM underestimate cylinder head temperature.

Common Causes

An internally failed temperature sensor can develop excessive resistance, driving signal voltage higher than the ECM expects.
A disconnected plug, damaged terminal, or corroded connector can interrupt the signal path and make the ECM see a high voltage.
A broken or chafed signal wire can create an open circuit between the sensor and ECM, particularly where the harness is exposed to heat and vibration.
A poor sensor ground or wiring repair can distort the voltage reference, so the ECM cannot interpret the temperature signal correctly.
An ECM fault is possible but uncommon; it should only be considered after sensor and circuit tests confirm the wiring and inputs are sound.

Model applicability: GM vehicles equipped with a cylinder head temperature sensor and a calibration that assigns P1289 to its high voltage circuit fault

Diagnostic Steps

Check the VIN specific service information first. GM code definitions and sensor equipment vary by engine, so confirm P1289 refers to the cylinder head temperature circuit on this vehicle before buying parts.
With the engine cold, inspect the sensor connector and nearby harness for loose pins, corrosion, heat damage, or rubbed insulation. These faults can interrupt the signal and create the high voltage the ECM is detecting.
Use a scan tool to compare the reported temperature with ambient temperature before starting the engine. A reading that is clearly implausible points to a sensor or circuit problem rather than normal engine heat.
If you have the correct wiring diagram and a multimeter, check the specified reference, ground, and signal circuits. Follow the vehicle service procedure; do not assume wire colors or voltage values are the same across GM models.
Repair damaged wiring or terminals, clear the code, then warm the engine while watching the temperature data for a steady, plausible change. If the signal remains abnormal, test the sensor and wiring to the ECM before replacing components.

When to Call a Professional

A technician should first verify the VIN specific GM diagnostic definition and confirm the vehicle is equipped with this sensor. Then compare scan tool temperature data with a cold engine and ambient temperature, check the sensor reference and ground circuits, and monitor signal voltage while warming the engine. A scope or meter test at both the sensor and ECM can separate a failed sensor from an open, high resistance connection, or harness fault.

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Commonly Asked Questions

Does P1289 always mean the cylinder head temperature sensor is bad?

No. The code indicates a circuit voltage problem under the applicable GM calibration. A disconnected plug, broken wire, or poor terminal can produce the same ECM reading, so test the circuit before replacing the sensor.

Can I keep driving with P1289?

Avoid unnecessary driving until you confirm engine temperature is normal. If the ECM is receiving a falsely cold reading, it may not recognize excessive cylinder head heat quickly enough to protect the engine.

Why does an open wire cause a high voltage code?

Many temperature circuits use a pull up inside the ECM. When the sensor or its wiring opens, the signal can remain near the pull up voltage instead of changing with temperature, triggering a high voltage fault.

Will replacing the sensor clear the code?

Only if the sensor itself caused the abnormal signal. The ECM needs to see a valid changing signal after repair; an open harness or damaged connector will keep the fault active.

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