ML 350 P0400 – EGR valve – Code clearing repair for free…almost

Alright, so here we are back on the grind on this truck.  Not that it’s been a bad car.  I cant complain, it’s actually been a great car to own.  Repairs have been few and always something I can fix myself.  Here’s the disclaimer guys, run for the hills if you like: here it goes.  I’m not a mechanic……. there I said it.

Dont email me upset if this shit doesn’t help.  This is not a diagnostic site, I’m not an expert, I simply post what has worked for me.  Most of my repairs have not gone over $150 bucks and that includes parts.

So..pheeeww…glads that done.  Lets get into this truck already.

I got a freaking check engine light last October, driving back from a Tae Kwon Do championship.  Great… I figured it’s something serious being that I’ve had the truck almost 10 years now.

Went to the local PEPBOYS and had it connected.  Good news is it’s a p0400 code and it has to do with the EGR system.  Ok so what’s the damage I ask.  We’ll upwards of $600 buck and that’s at PEPBOYS…NOT the dealer.

I asked will this make the car stop running?  Guys says no, it might affect your mileage but that’s it.  We’ll I figured I would check it out later.

After a few weeks of checking what it might be other than what the guys at PEPBOYS said it was, I came to find out it could be simple a vacuum lines, EGR tube or dirty EGR valve.  So here’s what I did.

Before I get to that, here’s what you’ll need:

All the images are clickable to see exploded views.


This is the scanner I used.

  1. Code scanner there’s many to choose from you can check them out here.
  2. All regular tools, no special tools needed for this repair.
  3. New Gasket, you can pick them up at the local dealer pretty inexpensive.

I went down to the local dealership and picked up what I figured I would need if it was just a simple fix.  Seals! that’s it.  I figured I might be able to use the same ones but what the heck the seals run for about $10 bucks why not change them. So you can see, I was really crossing my fingers and optimistic it was a simple fix.  After all the car was running fine.  No shaking, no smells or weirdness what so ever.

EGR Tube Gasket

12-egr tube gasket

11- egr gasket

EGR Valve Gasket

To start off I did the remove the air filter cover, removed the intake tube and connector till I could access the EGR valve area which really sound more complicated than it is.  I didnt even remove the water reservoir which made it a little more hassle to take out the intake tube but helped me avoid putting air into the cooling system.

I forgot to take pictures of all those steps so you’ll excuse me for over-looking those details.  It’s pretty self explanatory, just look around and you’ll figure it out.  Here’s how it went down.

Start by removing the cover to the air filter the rubber connector with the sensor on it and the plastic tube that connects to the intake manifold at the rear of the engine.      ……..   click images for larger views.

1-engine view2-connectors3-screw removal4-elbow connector
Once the intake is removed you’ll get a view of the EGR valve, EGR tube and the intake to the manifold.  

I initially thought that my EGR tube was clogged.  After hours reading online about what the problem could be that was causing the P0400 code.  I decided I would check that out first.  Fortunately the problem wasn’t a clogged EGR tube but that’s the actual reason I was looking into.  Again, being that the truck was running fine.

6-intake tube 7-egr tube 5-egr valve view

I didnt even remove the EGR valve till after I had removed the EGR tube and checked it out.  I cleaned out the little gunk that it had but a clean EGR tube was telling me it was probably the EGR valve.  Bummmmmmer…..

That’s when I removed the EGR valve to see about cleaning it or replacing it.  When I removed it I realized that one of the vacuum lines on the underside of the EGR valve was really bad and sort of had collapsed into itself.  Which was probably causing a blockage and keeping the valve from working properly.

That was the AHA moment………………… oh yeah!

I had a small piece just the right length and size for this.

8-base of egr 10-egr valve view 9-egr vac line problem

I forgot to take a picture of the new piece of vacuum line but that’s all I replaced.  NOTHING ELSE.

After re-installing the EGR tube and EGR valve, putting the intake back into place and reconnecting all the connectors and air filter I tried it out.

I cleared the code with my inexpensive code scanner and it’s been happy driving with no engine codes since then.


I tell you this, I really lucked out.  After all the reading I did it came down to these possible issues causing the p0400 code.

  1. EGR valve not working
  2. EGR tube clogged

I also found that there’s a small section of vacuum line to a small sensor at the front of the engine that could have caused the code.  I had replaced that part of vacuum hose before, which is why I had the vacuum hose in the first place.

I replace it and that wasn’t the problem so I had given up on it being a vacuum line.  I didnt realize that there was vacuum lines under the EGR valve that could be causing the problem.

I would have replaced the EGR valve but I didnt have one  and I only have this car to drive around.  Being that I figured it was the clogged tube that needed cleaning I didnt purchase one.  I probably should have replaced it being this one has almost 100,000 miles on it already but that’ll have to wait for another day.

That’s all DIYers….that’s how I cleared the p0400 code on my 2004 ML350.  If it wasn’t for the new gasket I purchased, it would have been a $1 dollar repair (just the foot of vacuum line).

Either way it was an easy repair.  Just stay calm don’t rush and get frustrated or worse give up.  I got it done and save $600 bucks in about an hour and a half.


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