P040C: EGR Temperature Sensor A Circuit Low

EGR temperature sensor A measures the gas temperature in the EGR path – for cooler monitoring, component protection and the EGR model. Whether it sits before or after the cooler depends on the maker: GM diesels have sensor A after and B before the cooler, Ford diesels sensor B after it. Like a coolant sensor it is an NTC, so a low voltage means hot. P040C means the signal is below its lower limit – very hot or a signal wire shorted to ground. Thresholds: GM's Duramax LML below 0.46 V for 5 s, GM 2014 below 0.14 V, GM 2018 a resistance below 404 Ω, Ford diesel below 0.69 V for 5 s; MIL after two trips. A substitute value is used and the EGR cooler check is suspended.

Medium severity. Can I keep driving?You can normally drive in moderation, but get it repaired soon: if the valve is stuck open, the engine may idle very poorly.

Symptoms

Check engine light on
Usually no change in driving
Substitute value used, EGR cooler check suspended

Most likely causes

Signal wire shorted to ground (chafed on the exhaust)
Sensor shorted internally
Water in the connector
Actual extreme gas temperature (EGR cooler faulty)

What to check, in order

Start with the simplest. You can tick off each step.

Check the connector and any vacuum hoses
With the connector unplugged, check whether the signal wire is making contact with ground.
Use live data to compare the commanded opening with the actual opening
Clean the valve and passages if they are clogged with carbon
Test the actuator with the scanner
Replace the valve if it still does not follow the command after cleaning

Good to know

Repair difficulty: easy
It affects emissions.

What each character of P040C means

P: Powertrain: engine and transmission
0: Generic code: same on every make
4: Emissions
0C: Specific fault in that system

Frequently asked questions

Can I keep driving with code P040C?

You can normally drive in moderation, but get it repaired soon: if the valve is stuck open, the engine may idle very poorly.

What causes code P040C?

The most common causes are: signal wire shorted to ground (chafed on the exhaust); sensor shorted internally; water in the connector.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.