P0478
Exhaust Pressure Control Valve A High
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.
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.
Related codes
Often seen with P0478
Read & clear P0478 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX