Automatic Climate Control Drifts Warm After a Radio Installation

The fault is mechanical/plumbing in the sensor's air sample path - not the sensor element and not the module.

Why each of the other candidates is excluded:

• Sensor element: its resistance matches the chart at the temperature it is reporting. A drifting thermistor fails the chart; this one passes. The element is reading the air that reaches it, and that air is wrong.
• Module: the module is doing exactly what it should with the data it has. It is being told the cabin is 68°F against a 72°F setpoint, so it commands a small amount of cooling and then trims toward that target. The measured 78°F cabin is a consequence of the bad input, not of a bad output command.
• Air distribution: the blend door tracks command exactly and all outlets are 45°F, so the doors and ducts are delivering cold air. Refrigerant performance is normal (235 psi high side at 95°F ambient is a healthy multiplier of ambient).

The 10°F offset between the sensor and the head-height thermometer is the whole complaint: the control head believes the cabin is already near setpoint.

Mechanism: the in-car sensor is fed by an aspirator - either a small electric fan in the sensor housing or a venturi tube driven by engine vacuum or duct pressure. A sample tube knocked loose, kinked, or left disconnected during the radio job means the sensor sits in dead air behind the dash and reads a stale, cooler local temperature instead of cabin air.

Confirmation

1. Watch live in-car temperature data while the blower is on high. In a working system the reading should track a thermometer placed at the driver's head within a minute or two. Here it will sit near 68°F while the cabin is 78°F.
2. Locate the sensor grille on the dash or steering column shroud and check the back side for the sample tube. Look for a tube hanging loose, pinched behind the radio chassis, or connected to the wrong port.
3. If the system uses an aspirator fan, command or power the fan and listen/feel for airflow through the grille; a seized motor is common after dash work.
4. Functional test: direct cold air (a/C vent or a cold pack) at the grille for 30–60 seconds. A properly sampled sensor will drop its reading; an unsampled one will barely move.

Repair

• Reconnect or replace the sample tube, or replace the aspirator motor/hose. Restore any mounting clips so the sensor sees cabin air rather than the space behind the dash.

Verification

• With the cabin at a known temperature, the scan tool reading must agree with the head-height thermometer within about 2°F.
• Set AUTO to 72°F at 95°F ambient, drive or load the cabin, and confirm the system holds within a few degrees instead of drifting 8°F.
• Re-check that no new actuator calibration codes appeared after the trim panels went back on.

Because nothing is electrically out of range. Fault detection for sensor circuits is built around opens, shorts, and out-of-range values; a thermistor reading 68°F with a resistance that matches 68°F is perfectly plausible.

• Rationality checks compare the in-car sensor against other inputs, but at 95°F ambient the cabin can legitimately be at 68°F, particularly during pulldown with the system on high. The module has no way to know the true cabin temperature.
• Cross-checks between the ambient sensor, evaporator temperature, and in-car sensor would only flag a gross inconsistency; a 10°F bias will not trigger one.
• Blend door feedback matches command, so no actuator or calibration code is set either.

This is why the diagnosis has to come from comparing commanded behavior to measured cabin conditions - vent temperature plus a calibrated thermometer against scan data - rather than waiting for a code. It is also the reason to always photograph the scan data before and after the repair.