P040B

EGR Temperature Sensor A Circuit Range/Performance

Emissions / EGR Generic OBD-II Medium

What it means

P040B means the EGR temperature sensor 'A' is reading a value that doesn't match expected conditions. This sensor confirms exhaust gas is actually flowing through the EGR system. A slow or out-of-range reading — often from carbon fouling on the sensor tip or a wiring issue — trips the code but usually causes no obvious driveability problem.

Medium

Can I keep driving? Safe to drive. This is an emissions-monitoring fault with usually no noticeable driveability issue, but it can fail an emissions test.

Real-world scan data

Seen in

989

scans in the past year

Found in

0.2%

of scans

0.3%

of scans that found faults

Appears alongside · share of P040B scans

P0401in 88.3% of scans

P0402in 50.6% of scans

P0071in 48.9% of scans

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

Common symptoms

What you'll notice

Check-engine light on
Often no noticeable change in how the vehicle drives
May fail an emissions/readiness test
Occasional pinging under load if EGR flow is affected

Common causes

Most likely first

Faulty EGR temperature sensor A reading out of rangeLikely
Carbon or soot fouling the sensor tip so it responds slowlyCommon
Damaged or corroded wiring in the sensor circuitCommon
Loose or corroded connectorLess often
EGR flow problem the sensor is correctly reporting (carbon-clogged passages)Less often

How to diagnose & fix

Work through it in order

Read freeze-frame data and note the EGR temperature the sensor is reporting.
Inspect the sensor connector and wiring for damage or corrosion.
Remove and check the sensor tip for heavy carbon buildup; clean or replace.
Inspect the EGR valve and passages for carbon clogging if flow seems wrong.
Replace the EGR temperature sensor if it reads out of range. Clear and re-test.

FAQ

Quick answers

Is it safe to drive with P040B?+

Yes. It rarely affects driveability, but it's an emissions fault, so plan to repair it before an emissions test and to keep EGR working correctly.

Can I just clear P040B?+

It returns once the ECM again sees the sensor out of range. Clean or replace the sensor and check the EGR passages first, then clear and re-test.

Read & clear P040B yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX