The Check Engine light came on today on my 95 E420 with 102K; car had just returned from a 30 mile drive with no problems and had been sitting for about 30 minutes. Car has been running fine, mid-20s MPG on the road but my wife complained last week it seemed to be rough idling at a stop light. A search of this site indicates the likely suspects are O2 sensors or possible vaccum line break.
What does it take to get a "code reading" i.e., what tool purchased where? Can the code be reset by merely disconnecting the battery and reconnecting? Where in the engine compartment? While I consider myself handy for most repairs I don't know smog stuff - is this something better left to my indy mechanic?
Finally, are O2 sensors something I should be replacing purely based on miles or does one wait until they fail? I'm thinking of replacing them even before the trip to the mechanic with the "universal sensors" mentioned by ke6dcj if that is the case.
Thanks for the help.
Cheers, Denny Argall

Denny, I wouldn't replace anything until you pull codes to see what turned on the light. Quite often, the codes are all the information you need to correct the problem. Maybe it's a simple fix. CHECK ENGINE lights are usually easy to diagnose and repair, as long as you've got the right information. If you can find out what codes are stored let us know. I'll bet it's no big deal.

That car should have a led/sw under the hood for both code reading and erase----Do a Search on flash code /led retrieve

Thanks for the encouragement and search tip - I will work them today and see about getting those codes read and will report back.
Cheers, Denny

I found the LED indicator in front of where the battery would be if it was in the front of the car - when I pushed the buttom the LED on #4 illuminated - no # of flashes, etc., just the number 4 slots lights up. Using JimF's link to a web site, I guess this means the "air injection system" is faulty, is that correct?
Searching some of Arthur's posts, I decided to check vaccum lines with my Mighty Vac - I cannot get vaccum on the main vaccum line coming out of the intake; I checked the EGR and it definitely is working (engine stumbled, etc.). I tried clearing the code with various methods mentioned in the posts (held the buttom for 4-30 seconds with the ignition on; held it down for 30 seconds with the ignition off) but no joy. Finally disconnected the battery and that cleared the check engine light.
Does it sound like a bad main vaccum line? Although the rubber ends look fine, I can't tell if the hard plastic line itself is bad (although being unable to get suction probably means it is bad) because it's sheaved in another plastic protector.
What I thought I might is just get a section of vaccum line temporarly between the intake and the "Y" joint over by the windshield washer tank. Any other recommendations?
I'm going on a little drive with my wife tomorrow and maybe I can get it to repeat the "roughness" she has described.
As always, I appreciate the help from everyone.
Cheers, Denny

<<
Location: Norfolk, Virginia
Posts: 8
DID SOME CHECKS
I found the LED indicator in front of where the battery would be if it was in the front of the car - when I pushed the buttom the LED on #4 illuminated - no # of flashes, etc., just the number 4 slots lights up.
>>

Do a search on code flash procedure . there are tons of them

I will try to explain it to you

First thing to know is that the Led is simply Mounted IN the #4 slot/socket...just where they put it..

So, now you turn key ON
Hold button and LED will come ON [ you got this far]
Hold for 2-3 sec
release button
Code Flashes will come back to led --count them
Repeat this procedure til the number of flashes come back to the first flash sequence [ in other words, every time you do this a higher number of Flashes will come back --ie 3, 7, 10, 21 ,34 ,etc
when 3 [ the first flash in the example] comes back , the computer has gone through the cycle and you are done.]
Write each one down..
Unforunately, you now have no codes cuz you erased them, but they will return
If you get 1 flash. there are No codes stored

Arthur - thanks for the quick reply - I will follow your instructions today and hopefully get back to you on the results. Thanks again for the help.
Cheers, Denny

Went on a 50 mile drive on the 14th - I didn't have any problems; same thing the next day; however, on last Monday my wife drives it and the Check Engine Light came back on. When I drove the car it seemed fine two day later. Since my wife was going on a trip, I didn't need the car, I thought I would check it further over the weekend.
When I went to read the codes, again there was one LED that flashed once - that was it. I tried two or three more times and each time only one flash - it just stay on as long as I hold the buttom.
Since I suspected the main vaccum line from the intake manifold that ultimately goes to the EGR might be bad, I plumbed a test line into that little "Y" manifold near the windshield washer bottle. Initially, I started the car and everything seemed fine (remember, I've never experienced the roughness my wife complained of). Today I started the car and it ran like crap - like there is water in the gas. The reason I mentioned water in the gas is because over the weekend my lawn mower was acting strange - I put in some fuel treatment and it straightened right out. We've had an ungodly amount of rain this spring and summer out here in the East.
Well gents - what do you think my problem is? When I call my wife tonight I plan to ask where she's been getting her gas but any recommendation on a remedy - a can or two of gas treatment or fuel injection cleaner? Any harm trying to run that 1/2 a tank through (I don't know why she didn't fill the car) - I hate the thought of draining the tank.
As always, appreciate the help.
Cheers, Denny

< >>

Re-read the code procedure..

The led lights as long as you hold the button down, but you only hold the button down 2-3 sec. - then you release it and wait -- that is when the code flash will come back...
if nothing comes back after release , you have erased the codes
There HAS to be a code if there IS a CE lamp on...

The led lighting when you hold the button down is just to tell you that you are making the initiation to retrieve codes ...wait for the REPLY...

For all - I believe the problem is solved and (embarrassingly) it appears to be as simple a spark plugs. With 42K on this set of plugs since the 60K service (which I had my mechanic in northern Virginia do) I just had assumed these plugs were good for 60K. I now know that the plugs are good old Bosch copper cores and that it was just the system telling me it was time for them to go. My Indie recommended doing the plugs before bringing it to him - thanks.
I did fix the vaccum leak which was the main hose from the intake over to the solenoid by the washer bottle that was hard & brittle where the hose was closest to the engine and "shielded" inside the black plastic wrap. I got back from 500 mile trip this weekend with ZERO problems, 27 MPG and renewed peace of mind.
Thanks everyone who volunteered info and in particular Arthur Dalton for walking me through the code procedure.
Cheers, Denny Argall

This is a prime example of how abusing your maintenance can cause you problems. You should be ashamed going 42k on those plugs.

If this had got to the shop and they had to diagnose your lack of service it would have been some real expensive spark plugs!

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Mea Culpa, Mea Culpa - but this wasn't a case of ignoring the service schedule; instead, I just did not realize plugs only last 30K - air filters, etc., were swapped out at 90K.
Notwithstanding my error, I still appreciate all the help I got from this site - thanks.
Cheers, Denny