P0478

Exhaust Pressure Control Valve A High

Q: Is it safe to drive with P0478?

Usually for a short distance, but reduced power or limp mode makes it unsafe in traffic. Get it diagnosed promptly, particularly on a diesel where backpressure control matters.

Q: Can I just clear P0478?

It returns once the ECM re-tests the valve unless the sticking valve or circuit fault is repaired.

Exhaust pressure control Generic OBD-II Medium

What it means

P0478 means the ECM sees a high signal from exhaust pressure control valve "A," which regulates backpressure in the exhaust (common on diesels for warm-up, EGR flow, and DPF regeneration). The usual cause is a valve stuck by soot or a fault in its wiring. You may notice reduced power, more smoke, or a limp mode, so it's worth diagnosing before longer drives.

Medium

Can I keep driving? Often driveable, but you may notice reduced power or the vehicle may drop into a limp mode — have it checked soon, especially on a diesel.

Real-world scan data

Seen in

213

scans in the past year

Found in

<0.1%

of scans

0.1%

of scans that found faults

Appears alongside · share of P0478 scans

P1002in 95.8% of scans

P0540in 95.3% of scans

P2604in 95.3% 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
Reduced engine power or acceleration
Possible limp mode
Increased exhaust smoke (on diesels)
Poor fuel economy

Common causes

Most likely first

Soot or carbon sticking the exhaust pressure control valveLikely
Failed exhaust pressure control valve or actuatorCommon
Wiring or connector fault in the valve circuitCommon
Loose or corroded electrical connectionLess often
Engine control module (ECM) fault (rare)Less often

How to diagnose & fix

Work through it in order

Read freeze-frame data to capture the conditions when it set.
Inspect the valve's connector and wiring for damage, corrosion, or an open.
Check the valve for soot buildup or sticking and clean or free it if possible.
Test the actuator's operation against spec; replace the valve if it fails.
Only replace the ECM if the valve and wiring test good. Clear and re-test.

FAQ

Quick answers

Is it safe to drive with P0478?+

Usually for a short distance, but reduced power or limp mode makes it unsafe in traffic. Get it diagnosed promptly, particularly on a diesel where backpressure control matters.

Can I just clear P0478?+

It returns once the ECM re-tests the valve unless the sticking valve or circuit fault is repaired.

Read & clear P0478 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