SM Auto

Vehicle: VW Golf | System: Exhaust Gas Recirculation | Codes: P0400, P0401

Overview

Symptom: MIL illuminated. Otherwise the customer has no performance issues.

Primary Cause: No EGR flow was detected by the ECU and a no flow malfunction was recorded. Another workshop replaced the EGR valve but a day or so later the MIL came back on.

Some other attempts were made to get to the bottom of the issue but without success.

When I received the car I could see a new EGR valve — brand unknown — and there was a new metal pipe that connects the EGR to the back of the intake manifold chamber.

The Fix: Looking at live data I could not see any EGR effect when the valve should have been open. After a few checks I could hear the valve open and close but it didn’t effect the flow. Stripping it down to see what was blocking or restricting the flow I noticed the gasket on the EGR valve to the exhaust side was on the wrong way around and actually blocking the flow.

1. Diagnostic Fault Interrogation

When the car came into the workshop it had been worked on elsewhere, first time it got a new EGR valve fitted and when that didn’t fix the problem a second one was fitted.

It then went to another workshop and it couldn’t be solved, I believe they fitted a new outlet pipe from the valve to the inlet manifold.

Carrying out a number of tests I could not see any flow when the valve was supposed to be open.

I confirmed using VCDS and an Oscilloscope that the EGR was receiving a command from the ECU.

One simple test of shop air directed back through the exhaust with a delta sensor in the intake manifold showed that when the EGR was commanded open it didn’t allow air through.

2. System Analysis

These systems are as simple as they come, a valve is commanded open to allow inert gas into the intake and because this is an SD engine it is monitored by the MAP sensor. When the gas is passed into the manifold the pressure increases so the vacuum decreases and the MAP sensor reports back to the ECU, the ECU is expecting to see this so as long as it sees that signal it’s happy. However if it commands the EGR and doesn’t see that feedback it will set a code, usually a P0401.

3. Repairing the fault

Not seeing any flow from my tests I decided that we had some sort of restriction even though all the parts were new.

I believed that I was going to find a faulty part — I highly suspected this no name EGR valve — but when I removed the valve assembly I immediately saw the issue — the gaskets which are not interchangeable were on the wrong way around and acting as a blanking plate basically.

4. Verifying the fix

After placing the gasket to its correct orientation I again tried my shop air blast from the exhaust system back through the valve. I commanded the valve open and saw my delta sensor fluctuate.

It didn’t do that before — obviously after what I found — but it was nice to prove my set up and actually see flow happen.

I test drove the car after the fix and it worked as it should. I kept using it until it set its drive monitor which was absolute proof that we hit the nail on the head with this weird one.

Such an easy mistake and an easy one to overlook unless you have a solid test plan.