Exhaust Gas Temperature Sensors: Why They Fail and How to Diagnose Them

What it is asked to do

EGT sensors are placed ahead of the turbo, ahead of the DPF, and ahead of or behind the SCR catalyst, depending on the strategy. The ECU uses those readings to decide when to inject fuel for regeneration, to protect the turbine from overheating, and to verify that the SCR system is dosing correctly.

A sensing element that survives this is not a thermocouple or an RTD in the conventional sense — it is usually a platinum resistance element in a housingspecified for the position, with a response fast enough to track a regeneration event that ramps several hundred degrees in a minute.

Why they fail in clusters

Regeneration is the stressor. During a regeneration the exhaust temperature rises by 300 to 500 °C over a few minutes and then falls again. Every cycle is a thermal shock. A sensor that has survived 2 000 regenerations has survived 2 000 of them.

When a DPF is chronically blocked — because of a failed EGT sensor, a failed injector, or short journeys that never complete a regeneration — the regeneration attempts multiply, and every temperature sensor in that exhaust is cycled far harder than it was designed for. This is why a vehicle that arrives with three EGT codes usually has one root cause and two casualties.

Diagnosis

Compare the readings of two sensors in the same exhaust path at a steady cruise. They should track within 20 to 30 °C of each other. A sensor that reads consistently 100 °C out has drifted; a sensor that reads implausibly high or low at all times is usually open or shorted.

Check the wiring before replacing the element. EGT sensor harnesses run along the exhaust, through heat shield grommets, and are exposed to more abrasion and radiant heat than any other sensor harness on the vehicle. A partially melted insulation that reads as a plausible but wrong resistance is a common finding.