P242C: exhaust temperature sensor 3 below its electrical range

P242C is the low-range electrical monitor for exhaust gas temperature sensor 3. The source covers both analog resistance sensing and a digital thermocouple arrangement. Those architectures must be distinguished before interpreting the reported value.

Architecture changes the units

For the digital thermocouple path, the table gives a raw temperature below −72.80°C. The analog row prints resistance below zero ohms, an unsuitable practical resistance-test limit without clarification. This article therefore does not convert that analog entry into a service specification.

The controller requires no cranking, valid supply, active run/crank, no diagnostic reset or communication loss, and completion of sensor initialization. It counts 19 failed samples out of 25 on a 100 ms task. Reporting is Type B, two trips.

Useful diagnostic decisions

Identify whether the installed system supplies analog resistance or digital temperature; testing it as the wrong architecture can misinterpret both data and wiring. A below-range signal is also distinct from a physically cold exhaust within the sensor's accepted range.

Verification should demonstrate that initialization completed and the qualified lower-range test passed. A missing digital message cannot serve as a valid low-temperature reading. Consult the exact vehicle information for the unresolved analog boundary and any electrical measurements before selecting a repair.

Source and application limits

This explanation covers the cited 2025 GM monitor. Confirm the vehicle and calibration in its service information before applying these limits; inclusion in this file does not establish model compatibility. GM diagnostic parameters, PDF page 1474.