After many years (and tens of thousands of HAPPY miles) the CEL on our (non-California spec) 1991 420SEL came on, stayed on, and then came back on the next time the car was started.

Starts OK, idles OK, runs OK... Light is on when running.

Car is at the Beach and approximately 180 miles from my most reliable and highly favored MB Indy shop. Don't especially wish to risk (?) the drive if by driving it that far I might exacerbate whatever problem there may be?

To my knowledge this car has never had the Oxygen Sensor (I do have a new one in my parts stash) replaced, certainly not in the last 15 years & 125K miles we have owned it.

I know this is not much to go on but any help or ideas as to a possible way to better diagnose would be much appreciated.

On the SC400 or the SVX I'd have some previous experience in coaxing a Code or two out of these pre-OBD systems: with the 420SEL I've had -0- problems so -0- experience. Don't even know if replacing the Oxygen Sensor would also clear the code if it actually fixed the problem... D(uh) U M B me!

Thanks in advance for any kind consideration!

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'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435

I would replace the Oxygen sensor as they are supposed to be replaced every 60,000 miles. Doubt that you would do any harm by driving your 420SEL a few hundred miles before doing so, considering that some folks drive a car with the CEL light "on" for thousands of miles because the engine behaves normally, and they get used to it being "on".

As you probably know, replacing the Oxygen sensor is a simple DIY task with the crow's foot tool.

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Fred Hoelzle

CEL has to do with emissions, not operational condition of the vehicle. If the O2 sensor has never been replaced, that'll sure do it. The CEL lights when the computer detects that something it is monitoring is "out of range". If the O2 sensor has finally crapped out (125K is pretty impressive!), the fuel trim will be out of range. Unless you're pinging or running hot, you shouldn't have any concerns getting home.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)