Left stranded by S500
 

My folks have a 2001 or 2002 S500 sedan that left them stranded by the side of the road yesterday. The car has about 70k on it now; we bought it new back in the day.

The check engine light has come on a couple of times in the last three months. The first time the dealer put in a new MAF unit. Just last week the CEL/MIL came on again, and they diagnosed that it needed new O2 sensors, so those were replaced.

Yesterday on a 1000-mile highway trip, the car suddenly started to stumble badly and lose power. Before my dad gave up, the car was going 25mph down the highway with a wide open throttle. He shut her down and had to be towed over 130 miles backwards to the nearest dealership.

This morning that dealer diagnosed a "complete catalytic converter failure," and had to order parts. Dad threw him the keys and rented a car to continue on the trip today. My best guess is that something was wrong with the replacement O2-sensor from last week's repair, and way too much gas got dumped into the cat -- ultimately burning it up. I'm still surprised that pretty much ground the car to a complete halt, though.

Any other theories? It looks like M-B is going to eat at least some of the repair, despite the car being out of warranty. Just wanted to hear ideas from you experts.

Cheers, John

Bump.

Dealer reports the car is repaired, but it'll be another week or ten days before it is picked up. Anyone have any thoughts or theories about whether they've gotten it right this third time?

Cheers, John

While living in Germany I always heard that S-Class Mercedes was superior in quality. Rumor has it that at German Mercedes factories only the best among auto workers are selected for S-Class manufacturing. Everyone chosen to work on it would feel privileged and proud.

Has it changed over time?

These silly comments have nothing to do with a cat converter failure nor, in fact, do they have anything to do do with automobile assembly quality, which is 99% design.

I toured the S-class production line last summer and most of the assembly is completed by robots. The few manual operations appeared to involve placing wiring harnesses and pounding in a trim strip on the roof.

The "guess" that the new O2 sensors ruined the cats is quite unlikely and they were probably failing anyway and the previous sensor diagnosis could have been erroneous or was in fact indicating a failing cat rather than a failing sensor. However, if the diagnosis is correct the fault lies with the repair rather than the design. Nevertheless at less than 80K miles/8 yrs the new ones will be free.

I was not referring to the specific case, but after much reading about S-Class quality issues and this one just fell in the that line.

Yes, it has changed over time.

My wife's 10 year old Japanese car still has none of the issue above, Cat converter and O2 sensor. Or maybe it has none of them :D.

What did they charge you in the end?

Looks like some one had fun driving your SL600 or impressing his gal with it!

lkchris -- thanks. Apparently cats fail frequently on M-B cars. I only own one, but my parents have half a dozen and love them. The S-class has been a remarkable vehicle in general, though lately it has had a few problems crop up. I get the distinct feeling that dealers are just throwing parts at it as temporary fixes, rather than taking the time to figure out the actual underlying problem. Believe me, I want to be wrong.

In non-M-B cars, every cat I've ever seen burn up was due to a bad O2 sensor causing an incorrect air-fuel mixture, ruining things downstream. In fact, I don't think I've ever seen one fail outside those circumstances, though I don't claim it's necessarily so here -- certainly seems coincidental, though. Most of these experiences for me are in gasoline racing applications, not daily drivers.

lmac -- start your own thread to warn others about another dealer -- it has nothing to do with a problem occurring in Texas!

Cheers, John

In my experience, the overwhelming majority of MB catalyst failures are due to the internals breaking apart. This usually results in customer complaints of knocking noises from the exhaust system, but sometimes the pieces will actually cause a clog, leading to drivability issues. It's extremely rare for a fuel mixture malfunction to cause catalyst damage for a couple of reasons. The various engine management systems that MB has used since the mid-nineties can trim the fuel mixture to compensate for minor problems. When the problem becomes more serious than it can adjust for, the CEL will come on, alerting the driver to a potential problem. Also, cylinder mis-fire detection is separated into two distinct categories. Catalyst Damaging, and Non-Catalyst Damaging. Problem cylinders can be shut down to prevent catalyst damage. Of course, if the driver ignores the CEL, or other symptoms, all bets are off.

The good news is, MB dealers have the ability to test catalytic converters to see if they're coming apart internally, before there's any outward indication of a problem. Whether or not this can prevent unnecessary O2 sensor replacement though, is another story.