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Old 12-08-2003, 01:00 PM
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Join Date: Nov 2000
Location: Alameda, CA
Posts: 45
W140 Clear scan codes FAILED

Can someone shed some light on this problem:

Last week I took my car (here in CA) to have it smog-checked and the check engine light was on requiring the station to clear the codes prior to running the test.
As such, they hooked up their CS2000 and pulled up a variety of codes that was causing the check engine light to come on. They found one source of the problem: the air hose on the air pump was half off it's mount. They corrected it, cleared the codes and restarted the vehicle.
The check engine light was still on and when they checked the scan tool for 'current codes', it still showed five or six live codes.
They cleared it severl more times, but the same thing kept re-appearing.

I decided to tackle this another day and went to work. 4 hours later I hopped in the car and the check engine light was OFF.

SO... I now run to the smog place and try to re-check. (Saving you the lengthy detail in the middle) After many attempts the car would not pass smog on the NEW (as of Oct 1) Smog test equipment requiring a Dyno test because the ASR senses the front wheels not moving while the rear ones are.... And that causes the car to go into Limp Home Mode.

1. Why did the codes magically disappear?
2. Any ideas on how to trick the car into allowing a dyno test?

Many thanks here in CA.

Joel P. Davis
1992 500SEL
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Old 12-08-2003, 02:40 PM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 18,745
Joel, I'm not sure what you're trying to accomplish by clearing codes. In most cases there's a fault that has to be remedied so the light stays off. Some faults do not keep the light on. The light will come on to let you know something wrong then the light will go off. Fault codes are stored in system memory so a technician knows it occured since the last time codes were cleared. Just because the light is off doesn't mean there are no faults the system is dealing with.

95 S420
87 300SDL
83 300SD ... $950, see cars forum
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Old 12-08-2003, 02:47 PM
Registered User
Join Date: Nov 2000
Location: Alameda, CA
Posts: 45
Sorry, I should have been more specific:
The codes activating the check engine light were two O2 sensor codes, an airpump code, and misfiring code, some emergency mode code, a cam timing error, and another I can't now remember.

It seemes pretty clear to both of us that the errors reported couldn't possibly be current as the vehicle was simply running too well to be producing so many fault codes.

What is strange is that they disappeared after the car sat for 4 hours. Or is this normal: the codes take time to erase themseleves?

92 W140 500SEL
Joel P. Davis
1992 500SEL
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Old 12-08-2003, 07:54 PM
JimF's Avatar
'94 S500: only 793 sold!
Join Date: Mar 2001
Location: San Diego, CA
Posts: 1,924
DTCs will . . .

NOT erase themselves in four (4) hours but will be removed from 'memory' after a number of 'trips'. A trip is defined differently for different year/models but in general, it's when:
1) the car runs for more than 5 minutes
2) speed is greater than 15mph
3) and a bunch of things I have forgotten!

So after 5 trips, the codes are removed if they do not occur again! Not all of the needed parameters but you get the idea.

Your car is exempt from the Dyno test since the ASR is NOT defeatable. My '94 S500 has the same ASR and it gets tested w/o the dyno.

Your SMOG station should have known that because of the non-defeatable ASR. In later models, the ASR was (is) defeatable.

I would demand that they test you for free since it's their error.
Regards . . . . JimF
'94 S500 Cpe

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Old 12-08-2003, 09:04 PM
Registered User
Join Date: Nov 2000
Location: Alameda, CA
Posts: 45

Your reply is extremely informative. Thank you. After I left the smog place I took it to a Mercedes only shop and they too didn't know why the DTC's were still in memory after re-starting the car. He diagnosed the car as most likely the wiring harness. (he might be right... just for the wrong reasons)

Just goes to show... it is really tough to find a '92-95 W140 expert; I can't tell you how many times I receive information or repairs based on total guesses.

The smog place didn't charge me and I will gladly go back and educate them. They're a good place and I take my less technical vehicles to them. I have an appointment with a smog referee tomorrow AM.

Joel P. Davis
1992 500SEL
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Old 12-09-2003, 02:22 PM
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Join Date: Aug 2002
Posts: 179
defeatable ASR

Is this also true with the 95 year S series (ASR defeat)?
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