What does it mean?
U1243 is a Network (CAN bus, module communications) fault code. It indicates: Data Circuit Message .
Symptoms
- Outside temperature display shows dashes, a clearly wrong figure, or stays frozen
- Ice warning appears in warm weather, or never appears at all
- Automatic climate control regulates poorly, especially just after start-up
- Automatic wipers or automatic headlights behave oddly on equipped vehicles
- Trip computer values unreliable
- Other network codes stored at the same time
Likely causes
- Ambient air temperature sensor open, shorted or drifted out of range
- Damaged sensor wiring behind the front bumper, where it is exposed to spray and stone chips
- Corroded sensor connector after washing or driving on salted roads
- The module that broadcasts outside temperature loses power, ground or its bus connection
- Break or corroded splice in the bus wiring, so the message never arrives
- Sensor refitted in the wrong position after bumper repair, so it reads engine or road heat
- Internal fault in the climate or body module receiving the value
How to fix it
- Read the reported outside temperature on the scan tool and compare it with an actual thermometer
- Locate the ambient sensor and inspect it and its connector for corrosion and impact damage
- Measure sensor resistance and compare with the specification at a known temperature
- Check the signal and ground wires for open circuit and short to ground
- Confirm the sensor sits in free airflow and not against a hot surface or the radiator
- Check bus continuity between the transmitting and receiving modules
- Replace the sensor if it is out of range; check module power and ground before replacing any module