HVAC Coolant Temp Sensor Short to Ground
The HVAC control module has detected a short to ground in the signal circuit of the heater coolant temperature sensor. This sensor, which is separate from the engine's ECT sensor in most Ford applications, measures the temperature of the coolant flowing through the heater core to regulate the outlet air temperature. A short to ground causes the module to read near 0V, interpreting an extremely high temperature or an open/short circuit fault, which disables automatic temperature control.
1. Commonly Affected Makes
2. Most Common Causes
- Heater coolant temperature sensor internally shorted to ground
- Signal wire (typically gray/red or similar per diagram) chafed or pinched against chassis/body
- Corrosion, moisture, or bent pins at the sensor or HVAC module connector causing contact with ground
- Faulty HVAC control module
3. Symptoms You Will Notice
- Automatic temperature control disabled
- HVAC system may blow hot air only or fail to regulate temperature
4. Step-by-Step Diagnosis
Check for diagnostic trouble codes and verify B1949 is present
Inspect the heater coolant temperature sensor connector for corrosion, moisture, or bent pins
Measure voltage at the sensor signal wire with key on; should be around 5V reference. If near 0V, suspect short to ground
Disconnect the sensor and check if voltage returns to 5V; if so, sensor is shorted internally
If voltage remains 0V with sensor disconnected, trace the signal wire for a short to ground or check the HVAC module
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, it is generally safe to drive, but the HVAC system may not control temperature properly, which could affect comfort and defogging ability.
How much does it cost to fix?▼
Repair costs typically range from $100 to $300, depending on whether the sensor, wiring, or HVAC module needs replacement.