P04A4

What it means

The exhaust pressure control valve B is mechanically stuck in the open position. The ECU commands closing for back-pressure assist or exhaust braking, but the position feedback shows it remains open - typically heavy soot, broken linkage, or a sticky valve shaft.

Is it safe to drive?

The vehicle is usually drivable for short distances, but the problem can worsen or affect emissions. Diagnose and repair as soon as practical.

Most likely causes

• Valve stuck open due to soot or broken linkage

  high

  One of the most commonly reported causes for this code — check this first.

• Carbon or debris buildup preventing free movement

  low

  Less common, but worth ruling out after the more likely causes.

• Component wear with age - mechanical play

  low

  Less common, but worth ruling out after the more likely causes.

Related help

• Full P04A4 diagnosis, causes & costs
• How to read check engine light codes
• Is it safe to drive with the light on?