Hello all!

I'm proud to be a new CLK430 owner, my second mb ever. At least, I was proud. I bought the CLK two months ago. Its a 99 with 51k miles - pretty good right? Anyway, I've put almost 5k miles in two months and about a week ago my "Check Engine" light came on. That day, I went to the dealer who informed me that my Mass Air Flow Sensor and Passenger side O2 sensor had tripped. They didn't just want to start replacing parts since it hadn't happened before so the reset it and sent me on my way. About two days later it tripped again; this time I went to a mom & pop mechanic who read the codes for free. This time it was two O2 sensors and no word from the Mass Air Flow sensor.

I'm clueless. I was going to just buy a new MAF sensor off eBay hoping that would correct the O2 sensor being out of limits but since it didn't trip on the second go-round, I don't know what I should do now. Oh, I have searched all over this forum for similar problems with no luck. I found one that said he had two tripped O2 sensors but there was no definite fix stated.

PLEASE HELP! I had that this happened less than two months after purchase, but I'm hearing bad things about CLKs...

 

Two approaches to MIL lights. 1. Keep on changing things "randomly" till problem solved (very expensive) or 2. try to figure out what is going on.
MIL illuminates if vehicle emmission levels exceeds legal limit. It monitors exhaust gas continuously via the O2 sensors. Even a transient condition can cause the light to illuminate dependent on all sorts of rules (dependent upon severity) etc in the OnBoard computer. It is even possible for the light to be turned of if the OnBoard computer detects that the fault has corrected itself (unlikely)
Need to know a bit more about this before suggesting a fix.
1. Since it has occurred several times is there anything common between the events. Was the engine cold, warm,accelerating hard etc? The approx engine temp, speed, revs, throttle position all help.
2. When the MIL was reset the garage should have read the OBD On Board Diagnostics Flash memory. This gives the conditions that caused the MIL to illuminate. Resetting the MIL clears this memory so you may need to get a repeat event. Specifically need to know what the engine fuelling parameters LTFT and STFT were doing when the O2 sensors set the light. If these are negative the chances are that the OBD was trying hard to lean the mix to keep emmissions within limit.
3. When connecting the diagnosics scanner it undertakes a "Component Self Test". Does this report C = complete and OK?
4. Not sure exactly what the service schedule is on a CLK430 but was a service carried out at circa 50k and should / were the plugs changed or other work undertaken on the engine. If plugs were changed were they the correct sort?
Answering these questions will give more insight into what is going on.

Chances are it is the MAF

but unless you like spending your hard earned cash I suggest you try to find out what is going on before telling the MB garage to fix it.
Is this a USA or UK spec vehicle?

Will be away over easter but will pick up the story again when I return. Pl copy your post also to asot53@dsl.pipex.com

Stuart

 

Stuart,

Thanks for the informative reply. The light has only come on twice so far. The first time I had just driven 4 hours and it went on when I was rounding the corner to my street. The second time was when I was sitting at a stop light, I had just put it in Neutral while waiting. So I guess both times it was while unloading the engine.

As for the diagnosis, the first time I got it checked from the dealer who just told me two fault codes occured: MAF sensor and passenger side O2 sensor. That was all they told me. Second time I got it checked from a small shop they wrote the fault code down: P0173 and P0170. They were nice enough to look up the fault codes for me which said it was a "fuel trim malfunction." I was a bit disapointed because I thought one of the codes was going to be the MAF.

As for service, I just bought the car two months ago. I'm not exactally sure what service was done on the car before I bought it from the dealer. The 430 didn't even come with an owner's manual much less any records.

That's about all I know for now. I had the light reset yesterday and I'm waiting for it to come on again. I did order a new MAF on eBay yesterday as well. I got it for $99 new so I couldn't turn it down.

Thanks so much for your reply. Its nice to know there is a place where a new CLK owner can go to get good advice!

-Chris

 

fuel trim malfunction was detected by the O2 sensor, but it might not be the O2 sensor problem.

it means that the engine was taking in unmeasured air...or vacuum leak.

if the engine is shaking, try to check the evaporative emission such as the regeneration valve.

 

I'm not sure if its lucky or not, but the engine runs like a top. No abnormal operation at all. Hope this helps :/

 

P0173 and P0170 ---- left and right cylinder bank partial load adaptation out of range, 95% of the time its MAF sensor. You'll need to have the adaptation of the ME-SFI control unit reset if you replace it. Don't be surprised if the O2 codes back up.