B1948: HVAC Coolant Temp Sensor Circuit
The B1948 code indicates a fault in the climate control coolant temperature sensor circuit. This sensor monitors the temperature of the engine coolant entering the heater core, allowing the HVAC control module to precisely adjust air blend and heating system operation. The code is triggered when the module detects a voltage reading outside the acceptable range—either too high (indicating a short circuit) or too low (indicating an open circuit). This code is commonly found in Ford and Lincoln vehicles, where the climate control system relies on this signal to regulate cabin temperature and prevent heater core damage from overheating.
1. Commonly Affected Makes
2. Most Common Causes
- Faulty or out-of-calibration climate control coolant temperature sensor
- Sensor wiring shorted to ground or voltage
- Open circuit in sensor wiring (cut or disconnected wire)
- Corroded, bent, or poor-contact sensor electrical connector
- Low engine coolant level causing erratic readings
- Faulty HVAC control module (uncommon)
3. Symptoms You Will Notice
- Inaccurate or no cabin heating
- HVAC system blowing cold air when heat is requested
- Erratic temperature control or temperature fluctuations
- Possible check engine light or HVAC warning light illumination
4. Step-by-Step Diagnosis
Check engine coolant level and top off if low
Inspect the coolant temperature sensor connector for corrosion, damage, or loose pins
Test sensor resistance and compare to manufacturer specifications
Check for shorts or opens in the sensor wiring harness
If all else passes, test or replace the HVAC control module
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, but the HVAC system may not heat properly. Driving with low coolant can cause engine overheating, so check coolant level first.
How much does it cost to fix?▼
Typically $100–$300 for sensor replacement, including parts and labor. Wiring repairs or module replacement can cost more.