Failed A/C low side temperature sensor (thermistor) inside the evaporator case; Damaged, chafed, or disconnected wiring harness at the sensor or along the evaporator case; Corroded or loose electrical connector pins at the sensor or module; Open or shorted circuit in the signal or ground wire between sensor and control module; Failed HVAC control module or BCM driver circuit; Moisture intrusion into the sensor connector causing intermittent signal; Aftermarket or incorrect sensor installed during prior A/C service

DTC codes / GM codes / P1531

P1531 · GMA/C Low Side Temperature Sensor Circuit

Q: What does code P1531 mean?

A/C Low Side Temperature Sensor Circuit. P1531 on a GM vehicle means the body control module (BCM) or heating, ventilation and air conditioning (HVAC) control module has detected an out-of-range signal from the air conditioning low side temperature sensor, which monitors the temperature of the evaporator outlet or suction line. The computer compares that signal against expected values and flags the circuit when the voltage stays too high or too low for too long, indicating an open, short, or sensor fault.

P1531 on a GM vehicle means the body control module (BCM) or heating, ventilation and air conditioning (HVAC) control module has detected an out-of-range signal from the air conditioning low side temperature sensor, which monitors the temperature of the evaporator outlet or suction line. The computer compares that signal against expected values and flags the circuit when the voltage stays too high or too low for too long, indicating an open, short, or sensor fault.

SystemPowertrain

Code typeGM specific

SeverityLow

Fix difficultyIntermediate

AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

Air conditioning may blow warm or not cool at all
A/C compressor may fail to engage or may cycle rapidly
Evaporator may freeze up, causing reduced airflow from the vents
Blower motor may continue running but vent temperature stays inconsistent
Often no symptoms besides the warning light, especially in mild weather
HVAC controls may default to a fixed setting or disable automatic climate control

Common causes, most likely first

Failed A/C low side temperature sensor (thermistor) inside the evaporator case
Damaged, chafed, or disconnected wiring harness at the sensor or along the evaporator case
Corroded or loose electrical connector pins at the sensor or module
Open or shorted circuit in the signal or ground wire between sensor and control module
Failed HVAC control module or BCM driver circuit
Moisture intrusion into the sensor connector causing intermittent signal
Aftermarket or incorrect sensor installed during prior A/C service

How to diagnose and fix it

Scan all modules and record freeze-frame data for P1531, then verify the sensor live data reading against actual vent temperature
Inspect the sensor connector and harness for corrosion, chafing, or loose pins, and repair as needed
Measure sensor resistance and compare to the GM temperature-resistance chart for the ambient conditions
Backprobe the signal and ground circuits at the control module to check for opens, shorts, or excessive voltage drop
Substitute a known-good sensor or simulate the correct resistance at the connector to confirm the module responds
Replace the A/C low side temperature sensor if resistance is out of specification or the signal does not change with temperature
Clear codes and perform an HVAC recalibration or relearn procedure if required, then road-test with A/C on max

What a P1531 often really means

A P1531 is frequently set by a chafed harness behind the glovebox or near the evaporator case rather than by a failed sensor, so inspect wiring before condemning parts.
On many GM platforms the low side temperature sensor is part of the evaporator thermistor assembly and cannot be replaced separately from the case, which changes the repair approach.
The code can set intermittently only when the A/C is commanded on, so a static scan with the engine off may show no fault until the HVAC module requests compressor engagement.
A weak or failing A/C compressor can cause the low side temperature to drop below the sensor's expected range and set P1531 even though the sensor circuit itself is healthy.
Aftermarket remote start or alarm installations that tap into the HVAC harness are a common hidden cause of this code on older GM trucks and SUVs.

Can I keep driving with P1531?

You can drive the vehicle safely with P1531 because it affects only the air conditioning system and does not impair engine operation or safety. Avoid running the A/C for long periods if the evaporator is freezing up, and have the circuit checked before summer heat makes the fault more noticeable.

Tools that help

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