P20E2: exhaust temperature sensors are compared after a long soak

P20E2 detects disagreement between exhaust-temperature sensors 1 and 2 in the cited GM 2009 Duramax example. The controller compares their startup readings after a long engine-off soak. It is not a general rule that two sensors at different exhaust locations must always read the same while the engine is operating.

The soak makes the comparison meaningful

The documented engine-off period exceeds five hours. The absolute difference between the two startup temperature readings must exceed 20°C, and the listed electrical range faults for those sensors must be inactive. The table describes evaluation once per key cycle and a diagnostic result on the first failed comparison.

Those prerequisites distinguish a possible biased measurement from the expected temperature differences that can occur in a flowing, hot exhaust system. A recent shutdown or an active sensor circuit fault changes the basis for this particular test.

Keep both startup readings

A technician should record the off-time history, both sensor values and related circuit faults. A single temperature reading cannot show which comparison failed, and disagreement alone does not identify which sensor is inaccurate.

The source supplies no instruction to replace both sensors or a particulate filter. It also does not establish identical criteria for later GM calibrations. Follow the matching vehicle diagnostic procedure to verify the temperature reports and reproduce the qualified comparison. Exhaust components may remain hot despite an engine-off state, so physical inspection requires the appropriate service precautions.

Source and applicability

GM 2009 09OBDG01 Duramax Engine Diagnostics, cited startup EGT1/EGT2 correlation monitor only. Individual vehicle, engine, wiring and calibration applicability has not been established from this parameter file.

General Motors source, physical page 21