Hi Everyone,

I have an issue I hope someone might be able to help me with. The Check Engine light in my 1999 ML 320 has come on intermittently for the last 3-4 weeks, and is now on again.The first time it came on I went to the Auto Zone, had the codes pulled and here's what they were:

PO170
PO173
PO141

The guy there said it was the oxygen sensor that is just beyond the Cat. in the exhaust system. Does that sound right? I have replaced the air filter, which was pretty bad, made sure the gas cap was seated well, put good 93 octane BP gas in it, all of the minor things that could be causing it. No offense to the Auto Zone, but I'm just not comfortable getting my car diagnosis there. I don't want to buy one oxygen sensor @ $159.99 and find out it's the wrong one (I think there are 4), or maybe it's the MAF sensor. Any input would be greatly appreciated.

BTW it has 68k on it and the performance has not changed at all. Still runs like a top.


Thanks in advance,

Chris H.

Ok heres what the codes mean..

P0141 O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
P0170 Fuel Trim Malfunction (Bank 1)
P0173 Fuel Trim Malfunction (Bank 2)

I guess the question is why is there no difference in fuel economy or performance? Also, why the intermittent on for a day, off for a week, on for a few days, etc...? Any help would be appreciated. Should I have posted this in the "Tech Help" section?

I too went to AutoZone and had the codes pulled for a CEL. I don't recall them right now but they were something related to left bank and right bank (I may have even posted them in a plea for help on one or more of the ML enthusiast sites)

AZ said O2 sensors and said which ones (there are four (4) I believe). AZ wasn't sure but they were eager to sell me either exact replacement or universal replacement "to save me some money".

Read everything I could find on the WEB and decided it probably was not the 02 sensors. Replaced the MAF with a screwdriver and 10 minutes.

CEL went out. Engine performing flawlessly...that was 11,000 miles ago.

Hope this helps.

__________________
Jim

Thanks Yucrew! That helps a lot. So you didn't see any performance issues either? Just the CEL? Since they are saying it's the O2 sensor after the CAT I am skeptical... I may just slap a new MAF in there and see what happens. Anyone else have any possible ideas?

No hints of any performance isssues....no starting issues, no hesitation, no misses....just the CEL on.

Don't ask about mileage....I typically don't pay much attention to it so can't give you much help there.

Shop around for the MAF (I was advised to be very careful on ebay).

I think I paid around $245....replaced with a 113 094 00 48 (Bosch 0280217810, same as original on the vehicle)....box received was marked 0280217811 but the part in the box was marked ....810.

www.boschusa.com says ....811

Good luck.

__________________
Jim

Chris -- your CEL issue sounds familiar. I too have a '99 320 with an intermittantly lit 'Check Engine' light. If you did a search on this site regarding the problem, you may have seen a posting from me on 04 10 09 responding to a thread started by Alan S entitled 'Check Engine Light on ML320'.

There is some similarity in our retrieved codes but despite the replacement of the MAF, the light has never stayed out for very long. Actually, it is unlit for longer periods when the ambient temperature is low, i.e., minus 25 degrees Celsius or so.

My point is simply that MAF R&R may not solve the issure for you. Short term saving of your old MAF in case the light comes back on might be prudent. It can always be put back into service -- immediatly or in the future.

Good luck

Geoff

Thanks Geoff for your input. I haven't changed the MAF yet because it was intermittent and I didn't want to just "throw parts at the problem". It has been on for a few days now, so I will just have to see what happens. I found one locally for very cheap I may pick up tomorrow. I hope it works.

Interestingly, the light seems to come on when the temperature is lower, not when it's higher as yours seems to. We had a cold snap to 10 degrees F and it came on then. If there is no change in performance, the car is running well, good mileage, runs cool, etc, is there any harm in just continuing to drive it? We have not been driving it much because of the issue.

Chris H.

Chris -- Gilly and others may well have another opinion but I have just continued to use the vehicle normally for the last few years. Neither the service departments at three different MB dealerships nor I have been able to permanently solve the problem and I need to use the vehicle.

I suspect that the danger in continuing to drive it is dependant on the fault that is triggering the CEL. I can see that a fault or an adaptation that causes the engine to burn rich for long periods of time would threaten the catalytic converters for example.

Sorry I can't be more helpful.

Geoff

Thanks Geoff. I have done some searches and found that:

1. Code PO141 is the passenger side rear O2 sensor, which may or may not actually be defective. It does not affect performance, and really isn't a big deal short term.

2. It might just be, as yours is, a glitch that cannot be determined. It runs very well as it always has. Gas mileage is fine, it runs cool, shifts and idles great, etc, etc.

3. It is no longer an "intermittent" problem. It's been on for a week. It was recommended that I use some octane booster and also possibly a bit if fuel injector cleaner and see if it might have been a bad batch of gas.

We will see, but anyway, at least it isn't stalling or running poorly. I would bet it's that O2 sensor.

Chris