P040A: EGR Temperature Sensor A Circuit

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. P040A is its general circuit code. No manufacturer document available to us uses it – GM and Ford report this sensor as P040B to P040E – so the description follows CARB's requirement to check input circuits for continuity and range. 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

Connector or wire at the sensor (heat, corrosion)
Sensor faulty
Engine module input

What to check, in order

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

Check the connector and any vacuum hoses
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 P040A means

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

Frequently asked questions

Can I keep driving with code P040A?

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 P040A?

The most common causes are: connector or wire at the sensor (heat, corrosion); sensor faulty; engine module input.

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.