What Exhaust Gas Temperature Sensors Monitor
Exhaust gas temperature (EGT) sensors measure the temperature of exhaust gas at specific points in the aftertreatment system — typically at the DPF inlet, DPF outlet, and SCR catalyst inlet/outlet. The ECM uses these readings to manage regeneration timing, confirm regeneration completion, and protect aftertreatment components from excessive heat.
Multiple EGT sensors are present in the exhaust aftertreatment system on most modern heavy trucks. Each has a specific SPN identifying its position. A fault on one sensor does not necessarily affect the readings from others.
EGT Sensor Fault Codes
EGT sensor circuit faults (FMI 3/4) indicate an electrical problem in the sensor or wiring. Out-of-range value faults (FMI 0/1) indicate the measured temperature is above or below the threshold the ECM considers plausible for the current operating condition.
An EGT sensor that reads low while the engine is at operating temperature is a common indicator of an open circuit — the sensor is disconnected or the signal wire is broken. The ECM substitutes a default temperature value for aftertreatment control calculations, which may cause suboptimal regen management.
Symptoms When EGT Sensors Have Faults
An aftertreatment warning lamp may appear with an EGT sensor code. If the sensor that monitors DPF inlet temperature fails, the ECM loses its primary indicator for regen completion and may extend regens beyond optimal duration or fail to confirm that a regen completed successfully.
Trucks showing abnormally long active regen times or failed regen codes alongside EGT sensor faults often have the regen management issue because of the lost temperature feedback.
Recording Guidance
Record which EGT sensor position has the fault (inlet, outlet, pre-SCR, post-SCR) — the position affects which aftertreatment function is impacted. Note whether aftertreatment warning lamps or regen request lamps are also active.
EGT sensor connectors in the exhaust path are exposed to extreme heat cycling and are prone to terminal corrosion. The connector condition should be checked before sensor replacement.
Safety Context
If the temperature monitoring system is compromised, the ECM may not detect a runaway regeneration event or dangerously high exhaust temperatures. Address EGT sensor faults before extended operation, especially on trucks in vocational duty cycles with frequent regeneration.
Related Pages
Sources
- SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium
Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - Cleaner Trucks Initiative and Heavy-Duty Engine Emissions Context United States Environmental Protection Agency · government · accessed 2026-05-05 · confidence medium
Source: United States Environmental Protection Agency, Cleaner Trucks Initiative and Heavy-Duty Engine Emissions Context. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
Does a Exhaust Gas Temperature Sensor fault code confirm a failed component?
No. The code identifies the monitored parameter and the type of condition detected. Wiring, connector corrosion, related system conditions, and calibration effects can all produce aftertreatment codes without the named component failing. Confirm with live data and OEM service information before replacing parts.
Will a Exhaust Gas Temperature Sensor fault trigger a derate or inducement?
It depends on the fault type and calibration. Some aftertreatment codes trigger immediate torque derate; others escalate after an operating-distance threshold. Check active vs. stored status and look for related inducement or derate codes alongside the main fault to understand the urgency.
What tool is needed to diagnose Exhaust Gas Temperature Sensor faults?
OEM diagnostic software (Cummins Insite, Detroit DiagnosticLink, Volvo VCADS Pro, etc.) is needed for live sensor data, temperature history, SCR efficiency data, and dosing event logs. A standard J1939 scanner reads the SPN/FMI but typically cannot access the full parameter set needed to distinguish a sensor fault from a system efficiency fault.