91 300E, 02 sensor code, repl. 02 sensor, ECU, still get code

[kristian85]

Hello,

Check engine light is on, code 17 (02 sensor), replaced sensor, light still stays on. Swapped ECU, still somes up code 17. For some more history, I quote my earlier post: Blown headgasket, went in for repair, crack in head not noticed after head rebuild, coolant got into exhaust, head got welded/fixed, headgasket problem now completely sorted, but check engine light came on, indicating code 17, bad 02 sensor.

For the first 1-2 days after repairs, with exhaust relatively full of coolant, car had some trouble coming up to proper idle speed after coldstarting--it would barely fire, chug along at 400 rpm, then idle nearly normally after 30-45 seconds, although it would still hesitate on larger applications of throttle until more warmed up. These symptoms went away after 100 miles or so, and car ran increasingly better.

Then, shop replaced 02 sensor, check engine light still came on, code 17. Shop did M-B fix of adding another ground to old 02 sensor, checked + adjusted fuel mixture with C02 meter, everything seems fine, car runs fine, but check engine light still comes on, still gives code 17. check engine light wasn't on before headgasket repairs. Car now starts fine cold, starts well hot, runs great.

Wiring, yes? Any good ideas, hints?

Thanks!

Kristian Soholm
Seattle

[kristian85]

ping

[stevebfl]

Fault codes are set for specific reasons. A fault code for an O2 sensor is usually set for its inability to light off within a certain time or for switching too slow, dwelling rich or lean for too long. Can you test this?

If you understand what is faulted, testing simply identifies the fault. For those reading codes and replacing parts. Hope you have deep pockets. One of the greatest goofs in the auto tech world is the free AZ code reading. Talk about useless information. A fault code without enabling criteria or the ability to test such is a parts store's dream.

[kristian85]

Thanks for the response--

We switched the 02 sensor with a new one, and it still gave 02-sensor fault--that's what puzzling. So, between swapping 02-sensor and ECU, you'd think you had eradicated possibility of faults with either of those units.

PS: Deep pockets--what do you mean...?

Thanks again,

Kristian

[stevebfl]

Deep pockets implies lots of money. Have you ever heard the tale of the three blind men and the elephant. Each grabs a bit of information and creats a picture, each wrong.

You are approaching the problem like a blind man thus you will need deep pockets. Your question and problem is so general that it is hard to help you without trying to give you some perspective.

DO YOU UNDERSTAND the enabling criteria for the code that has been set?? An O2 sensor code is like saying my car is running bad. Everything causes O2 sensor codes!!! Any fault in control causes them. to analyze the problem one must first know the enabling criteria that was faulted. As I stated in the first reply O2 sensor get faulted for a number of reasons. The most common is that the sensor voltage stays at one voltage toooooo long. If the code says that, then the first test is to watch the O2 sensor voltage. It will be pegged rich or lean. You need to understand closed loop operation and verify it. If the problem is that it cycles too slow (the code will tell one this - there are more than one O2 sensor code in other words) then one should look at the heater circuit.

The codes give you a start, if the next step is the parts counter you will need deep pockets.

[kristian85]

1. Obviously the term "deep pockets" is known. I was asking a different question.

2. AFAIK, this forum was set up for non-mechanics, as in "blind people", to ask technical questions of "experts", no?

Thanks again for the response.