kev

hey everyone,

i have a 2001 c240 that's been throwing a check engine light for a couple of weeks now. car seems to run smoothly, although since it's not mine i have no idea what's normal, what's not (it's my cousins', borrowed while he's out of town, and since i'm better with cars than he is, thought i'd try and save him some at the stealership and fix it myself).

well, i scanned the codes it was throwing with a regular OBDII tool:

P0175 - Fuel trim malfunction (Bank 2)
P0170 - Fuel trim malfunction (Bank 1)
P0115 - Engine Coolant Temperature Malfunction

i'm guessing this list is reverse (as in, the first code detected is the last one listed here), but i'm not entirely sure what to make of it. sounds like either the thermostat, the sensor, or (please please no) one or both of the O2 sensors. what's weird though is that the CEL turns off for a day or two at a time then comes back on.

any insights? i was thinking i'd start with the thermostat and coolant sensor since they're the cheapest, but any guidance would be appreciated.

thanks all,
-Kev

vraa

I just checked through https://mbworld.org/forums/mercedes-tech-talk/23372-check-engine-codes.html

Try a new mass air flow sensor or clean your existing one if it's possible. I know I can do it using alcohol on my old LS400, see if it's possible with your car if you can narrow it down further and make sure that's the problem.

MBtech1

95% of the time if you have the adaptation codes for both banks like you do, its a bad mass airflow sensor. Unfortunatly they are about $350 but you dont have much of a choice if you want the light turned off.

kev

Hey, thanks for the replies guys. I was really worried it might be the MAF or O2 sensors; I can't believe how expensive those are for Mercedes.

One thing is, though, from the post ricky linked to, and the google searches on these codes I've done, few people seem to have that third code -- the coolant temp malfunction. And since it was the first code reported (and hence the last on the list), I wonder if that's a/the bigger issue? I can look up the engine temperature on the speed cluster computer, so it seems like the thermostat's working okay, though.

Anyone with experience on cleaning MAFs on the w203? I haven't even looked inside yet to see if I can locate it. Also, changing coolant temp sensors/thermostats?

Cheers,
- Kev

MBtech1

Its funny, out of all the time I have worked on MBs I have never seen that code come up on any cars. There are different temp sensors for the gauge and for the ECU so its possible that the coolant temp sensor that is for the ECU could be malfunctioning. If I had the car in front of me with a Star Diagnosis computer attached, it would probably be pretty easy to pinpoint.

kev

>> I have never seen that code come up on any cars

haha. lucky me. well, i've searched around the forum, and found instructions on how to clean the MAF, so i'm going to try that first, see if it clears the codes temporarily at least. that way i'll know if it's the MAF that's done it. i finally got a chance to look under the hood, and the airboxes were filled with crud; i think it's pretty likely something got through and f-ed up the MAF.

as for the other code, do you (or anyone) have any idea how a) i can figure out which coolant sensor i should order and b) how easy/hard it is to replace? i checked out mercedesshop.com (recommended on this forum) and they have two parts that seem like the right one for this: a coolant level sensor and a thermostat kit.

thanks for all your help, i really appreciate it.

-kev

MB_E430

Hey Kev do you have the link for cleaning the MAF I did a search and found nothing? I checked my codes this weekend and got the PO170 and PO173 left and right bank and from the threads it points to the MAF.
Thanks!

kev

I found it on this thread: https://mbworld.org/forums/c36-amg-c43-amg-w202/108994-maf-problems-cleaning.html

I haven't had a chance to do it myself yet, but read the whole thread before you attempt it; sounds like it's a temporary fix at best, and it's quite possible you may end up making things worse.

hope that helps.

speaking of help -- anyone? help?

-kev

sdanville

I installed a new MAss air flow sensor..to cure misfires.
I had PO300 and PO301 to PO306. codes. ( all cyl misfires)

After the new Bosch Sensor was installed It ran just the same and
i check the codes again. This time they came up the same PLUS PO170 and PO173.

I replaced the 12 spark plugs ( replaced the bosch platinum 2's with only 20,000 miles ). Only tested one mile and it was better. Will update this thread after a longer drive.

Serndipity

Here is one of the best links regarding corecting MAF problem.

https://mbworld.org/forums/mercedes-tech-talk/147337-how-clean-maf.html

In summary, taking the MAF out to clean or replace is a 5 minute job requiring only a screwdriver.

Often it can be cleaned....but don't use Q tips etc. They can easily damage the element. Use the CRC aerosol spray referenced in the above post.

The CEL will turn off after 3 cold start cycles. However, it takes 200+ miles of driving to clear all of the OBDII sensors from 'not ready' to monitor to 'ready'.

Most states require no CEL on and OBDII sensors in the ready state to pass inspection, so if yours is coming up, don't wait until the last minute.