P242B: exhaust temperature sensor 3 fails to respond after a soak

P242B is a range/performance diagnosis for exhaust gas temperature sensor 3. This GM strategy compares a maximum temperature reached during a calibrated period with a baseline captured after an engine-off soak. Insufficient difference indicates an unresponsive temperature signal.

Why a plausible reading may still fail

The source says the test can detect a sensor outside the exhaust pipe or information stuck for another reason. Either condition can leave a believable number on a scan tool while preventing the expected response. The threshold is a calibrated map based on the captured temperature, not a single universal temperature-rise figure.

The engine-not-running timer must exceed the printed 10,800 time value and remain valid. After restart, sensor initialization must finish. Electrical-range, quick-change, communication and key-on faults are exclusions. The test has no additional debounce after qualification and is Type B, two trips.

Verification

Compare the captured baseline and subsequent maximum, not two arbitrary readings taken during unrelated operating states. If the soak timer or another EGT fault prevents the monitor, no result does not establish a repaired sensor. Use the actual calibration's procedure to reproduce the qualified test. The summary does not provide a universal soak-and-drive recipe or prove which of its possible unresponsive-signal mechanisms occurred.

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 1472. GM diagnostic parameters, PDF page 1473.