What are EGTs on a diesel? EGT stands for Exhaust Gas Temperature. The EGT sensor is responsible for reading the temperatures of the aftertreatment system components such as inlet DOC, inlet DPF, outlet DPF, inlet SCR, outlet SCR, etc., and sending this information back to the ECM to maintain proper temperatures that control the aftertreatment system to reduce the harmful emissions.
When an EGT sensor fails a check engine light comes on, then you troubleshoot the problem. The problem can result in conditions like a clogged DPF, check engine light, DPF not regenerating correctly, and other issues. Normally in the aftertreatment system, it's common to have 3 or 5 EGT sensors, so the ECM knows when and how to manage regeneration and SCR operation.
A common question is: how to read an EGT temp sensor. The answer is, EGT sensors are read by the ECM only. If you're looking to verify the operation of an EGT sensor... EGT sensors are "thermistors" - an electrical resistor whose resistance is dependent upon temperature, either increasing resistance or decreasing resistance. Thermistors are either NTC (Negative Temperature Coefficient) or PTC (Positive Temperature Coefficient). So the real question that should be asked is "how do I test an EGT sensor", not how do I read it. You read it with a scan tool; which is an OEM or aftermarket device that allows you to communicate with the vehicle's ECM.
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