Fault Code 4 Keeps Coming Back w140
 

Does anyone have any suggestions on what items to check for a persistent Code 4 showing up? I have checked the vacuum hoses in my W140 '95 and found one that was disconnected several weeks ago, which I think caused the CE light to come on. I retrieved several codes via the "blinks" on the red light retrieval window. After checking and fixing the vacuum hoses I cleared all the codes as described by JimF and all codes went away. A few days later, CE light comes on again and gives 4 blinks. I've cleared the reader several times now and the CE will eventually come back on and give 4 blinks on the LED.

There are no symptoms that I can tell on the engine. The engine idles and runs well, as usual. It's just that the code 4 keeps showing up.

Any ideas? Thanks!

Couple of questions - which engine? When asking an engine question it's a good idea to say which engine. Also it's not clear - are you reading all the codes or just the pushbutton/LED codes? If you're just doing the LED the next step would be to read the rest of the codes on the 38 pin connector.

Sorry. It is a 1995 S420, so it is a 119.xx engine. Yes, I am reading the code by simply counting the LED lights.

You need to build Arthur Dalton's code reader and read the other pins. Try a search. It's very likely you will find more clues on the other pins. If the car is running fine just read all the codes, erase them and see what comes back.

Does the AIR pump operate when the car is 'cold' . . when you start it?? It should run for a few minutes. If not, there's a good place to start.

Ok will check operation of the air pump as soon as I locate it!:)

It's on the left side as you look from the front of the car below top cover.

You should be able to hear it as it pumps extra air into the exhaust; it's pretty noisy.