Chedeng88

I got a CEL the other day and this is what I pulled from my OBD II scanner.
My understanding is that it's for lowering the combustion temperature a bit by recycling some of the exhaust gases.

My C55 seems to be running fine. No rough idling, no overheating, no hard starting, acceleration is smooth etc.

How critical is it? Can I get away driving the car for a few more days without having this addressed?

Is this usually caused by a clogged vacuum hose?

C230 Mulac

You will be fine, it's one of your EGR components, EGR valve, EGR change-over valve, etc. Not the end of the world. Did you try clearing the code and see if it comes back? Sometimes the EGR valve can get stuck open/closed and throw the code. These EGR codes can also sometimes be thrown if the gas cap is accidentally left off or you didn't tighten it properly.

In any case, it's not major.

Chedeng88

I was planning to clear it this weekend but the CEL disappeared by itself the other day.

It was probably something stuck or clogged in the EGR system that got freed by itself right?

Anything else I need to do, like clear it from the stored codes to eliminate any potential issues with emissions testing?

ghiaguy

Unless you have an impending smog check, the codes should work themselves out in about a 100 miles of CEL free driving. I think you are good unless you get the light back or there are other symptoms.

Wodeshed

I have the same issue; smooth running car, no idle problems, but a p0400 code that keeps coming up after I clear it, usually within a few hours of driving.

Not sure if it's a vacuum line with a leak or a clogged EGR line "pipe", as I've seen discussed in the ML 320 forum. Anyone have any idea if the clogged pipe phenomenon can coexist with a smoothly running car?

Incidentally, I'm in Florida...land of no emissions tests, so it's not as though I need to fix it immediately to pass an inspection.

Supgee

Might be worth a shot to get an injector service done... they remove the fuel pump fuse so the engine doesn't run... and then they hook up this pressurized cleaner to the fuel rail and start the car... the engine runs on this cleaner solvent till it chokes out... works really well.
We use it at our dealership, and I have personally seen before and after flow tests on "bad" injectors... and its restored them almost 100%... also with the diesel engines they make a special EGR cleaner that cleans'er right out. The engine light may come on for low fuel pressure as the engine slowly chokes out but its no biggie.. plug the fuel pump back in and clear the code and she's ready to go

Viper98912

i just had this happen as well. Cleared and came back. Most of the time I have a smooth idle but occasionally gets rough. Took off the airbox one afternoon to take it apart but decided to put it back on when I saw how close this thing is to the firewall. I'll have to take it apart to investigate when I have a little more time.

Chedeng88

Just an update: I initially cleared the code and it came back after a week or two. After a few cycles of clearing, I got tired of it and just let it be. Then it actually went away on its own after a few days. So now as long as it's just that code, I don't even bother clearing it anymore and sure enough it goes away on its own only to come back a week or so later. It has been that way for the past year and the car runs smooth, no issues or other symptoms.

I wonder if its really safe to just leave it this way and just deal with it when my next smog comes near (which is still a 3 years down the road)?

Frederic944

EGR is only there for emissions to, as you mention it, lower the combustion and thus reduce the NOx, EGR not working is actually better for performance.

Viper98912

I got tired of the CEL coming back (plus I had to pass emissions) so I went ahead and replaced the EGR and tube earlier this year. A little bit of a PITA due to how close it sits to the firewall. Regardless, got it changed out in a Saturday afternoon.

Only thing that stunk was that my MAF was on it's last leg and through removing it to access the EGR, it went down the toilet.