What does it mean?
B1242 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Evaporator Sensor Open (High) .
Symptoms
- The AC compressor does not engage, or air conditioning appears to be disabled altogether
- The evaporator temperature in live data sits at an implausible value and never changes
- Vent air stays warm even with AC selected
- On some systems the compressor runs continuously and the evaporator freezes, cutting airflow
- The AC indicator on the climate panel may flash
Likely causes
- Open in the evaporator sensor signal or ground wire, letting the module input float high
- Sensor probe pushed out of the evaporator fins or its connector disconnected
- Internally open thermistor inside the evaporator sensor
- Corroded connector at the HVAC case caused by condensate
- Wiring damaged where the sensor lead exits the heater box
- Failed input circuit inside the climate control module
How to fix it
- Compare the evaporator temperature in live data with the actual vent outlet temperature
- Locate the sensor probe on the evaporator core and confirm it is seated in the fins and plugged in
- Jumper the connector pins and watch whether the live value jumps, which proves the wiring and module
- Measure the sensor resistance at a known temperature against the thermistor specification
- Repair the open wire or corroded terminal and protect the connector from condensate
- Replace the evaporator sensor if it is open, clear the code, and verify the compressor cycles correctly