P242E: exhaust temperature sensor 3 changes implausibly fast

P242E is a quick-change monitor for exhaust gas temperature sensor 3. It evaluates the magnitude of a raw temperature difference and fails above 100°C. The source describes internal sensor faults, deteriorated wiring and connector electrical issues as failure modes that this dynamic check can detect, without ranking them by likelihood.

The check needs a qualified signal

The counter is 12 failures out of 25 samples on a 100 ms task. Run/crank must be active and valid, the engine must not be cranking, and diagnostics must be enabled. Communication loss and electrical faults are exclusions. The unfiltered temperature must lie within the stated 140°C to 1,070°C range, and initialization must have completed.

Why a normal average is insufficient

A slowly refreshed or averaged display can hide the step changes used by this monitor. Preserve the available raw history and compare the fault's timing with sensor status. A steady but offset temperature is a different behavior from the rapid variation described here.

Verification should establish a completed quick-change check during eligible temperature conditions. A cold-start scan outside the enabled range cannot demonstrate a pass. The code does not by itself choose between the listed electrical mechanisms; the specific circuit procedure remains necessary. Reporting is Type B, two trips.

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 1476. GM diagnostic parameters, PDF page 1477.