Hi,

I have a E420 with 128000 miles the check engine light keeps coming on and off. When on it stayed on for a while and when off stayed off for a while. Lately it is on and wont turn off. Using the push buttons I read code 4 something to do with air injection. The air pump seems to work ie. it comes on when cold and shuts off after a while. What is code 4 and how do I diagnose this problem. I have the home built code reader too so probing pins would be no problem.

The second problem is with the climate control. The ac works fine. Just recently it started to get cold and temps dipped into the low 50's. When I set the interior temperature to 75 and set fan speed to auto the air that comes from the vents is not warm at all even after the car warms up. I don't know if it is related but one thing I noticed is that on a hot day (90+) when the ac is on and the car idles a lot (30 min +) occasionally the side vents blow hot air in. It keeps blowing for a while and then switch back to normal.

Thank you.

Hi,

Code 4: "Air injection system faulty hot film mass air flow sensor with hot wire"

Unfortunately, I think that's bad news and probably de air mass meter is starting to fail and that's expensive http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=26H19PM5K26H19PPRQ&year=1994&make=MB&model=E-420-001&category=B&part=Air+Mass+Meter



I've read you can clean it using some product (electronic contact cleaner) but I have never tried it. Don't touch the film inside the meter. Is the car running ok? You could try disconecting de meter with the engine running and observe for changes.

Did you delete the code to check if it comes back?

I hope this helps.

__________________
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Juan
1995 E320 Sedan
2008 Ford Escape
RC Helicopter

That's the code for an E320, not for an E420. For a 420 it's simply "air injection system faulty" - has nothing to do with the air mass sensor. It's telling you to check the LH-SFI / pin 4 for more codes.

Make sure that "#4" is the only code in the DM.

As deanyel says is the AIR system; check that the vacuum line hasn't cracked or slipped off the main pump or the switchover valves . . it's a common occurance.

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Regards . . . . JimF
-------------------
'94 S500 Cpe

Visit my Mercedes Web Page

If you have good vac source and normal AIR Pump operation, then the code can be caused by a weak 02 sensor,
The trigger circuit for AIR code is the ECU does a Self-Test and looks at the 02 sensors output to read a VERY LEAN condition.
This common fault is the 02 is no longer capable of such a low {Extreme} reading and the ECU sees this as a Fail and brings on the code.
This takes a lean condition voltage test from the 02 to verify.
A Commonly over-looked diagnosis.........

I second that opinion. I had a similar problem with my 300E 2.8. The Check Engine light would be intermittent. So after throwing parts at it, it turned out to be the O2 sensor. They run about $350 for the 2 of them (post and pre cat. converter).

How is your gas mileage? Have you noticed a lack in power on occasion, mostly when acceleration from coasting???

-Mike

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You must be thinking of another car - a 93 2.8 has only one O2 sensor - as does the E420 in question. Should run about a hundred dollars or a little more.

Thanks for all the replies.

As suggested I read the codes on pin 4 LH-SFI module and got back a 1 which means no fault. The O2 sensor was replaced about a year ago.

Can it go bad so soon like this?

As to my gas mileage I get about 270 miles to a tank. That is with stopping the pump when it click with no overfilling as I found it to be impossible with this car until the low fuel light comes on. I drive 75 % highway with rest in the city. I only use 93.

Is that good mileage?

While I was reading the codes I also probed other pins and on pin 7 EA/CC/ISC I get code 6 - Starter lock-out/backup lamp switch (S16/3). I don’t think this is relevant to the problem I have but I will look into it as well.

I checked the vacuum lines that go to the pump and all is ok. I also checked the other ones that I could see and inspect and they were ok. I had changed the elbows, the ones I could get to, about 7 month ago. The switchover valves I could not find.

Where are they located?

What do I look for?

Does the pump coming on mean it is ok?

Thanks

.....ooops. Sorry!

__________________
-----------------
Juan
1995 E320 Sedan
2008 Ford Escape
RC Helicopter

Any help with post # 8 and second part of post #1?

Thanks

Tried to find a picture of one but not luck. For my car, there's two (2) S/O valves located in the passenger highlight area. Have vacuum lines going in and out.

There's the main vacuum line going into the AIR pump and it's is notoriously knocked off!

The AIR working means it's 'ok' as far as I know.

__________________
Regards . . . . JimF
-------------------
'94 S500 Cpe

Visit my Mercedes Web Page

LATE UPDATE:

It turns out one of the diverter valves was bad and the pipe that connects to the pump was cracked. After replacing both of those items everything is back to normal. The hot air on the side vents is still a TBD.

FINAL UPDATE: The hot air from side vents during A/C issue turned out to be the monovalve. I ordered the monovalve and the one I received was not for an E420. The original had 4 ports and I received the one with 3 ports. I rebuilt the original using the parts from the 3 port monovalve I received. The original would have cost $250 and the one I got was like $100. So it all worked out in the end. Now I have ice cold A/C. I took pictures of the whole process so if anyone needs it I can PM them. Thanks for all the help.