OT: 1993 Audi 90. 02 sensor control - post cat

[talltorontoguy]

Hoping someone might have some thoughts on this one.

This car (2.8l v6) requires 4 02 sensors, 2 pre-cat and 2 post-cat. The 2 post cat are actually 02 control sensors that override the two pre-cat sensors.

Anyway, the pre-cats can be replaced with bosch generics, or the ford mustang ones like we do on E34's.

I can't find a replacement part for the two post-cat ones. They're 4 wire sensors, but I don't know if a generic 4 wire will work or if there is a special requirement. The dealer wants over $250 each for these... so I need help.

Let me know if you find something,

Rod

[winfred]

usually it's the pre cat sensors that the computer adjusts from, the post cat sensors are just there to check cat condition, theres two different types of sensor that should be used in that general year range of euro cars, one makes it's own voltage (easy to check with a graphing meter and propane torch, and my bet that's the type used on that car) and the other manipulates the voltage signal from the computer. with a graphing meter set to a 1 dc volt hit the sensor with the torch on a decent flame, with in about 30 seconds it should be over a half volt, after a minute or two of heating the sensor you can start playing with it removing the flame the voltage should drop off sharply and to near zero with in under a half second (my snapon vantage says 10th of a second) on the graph it should go near straight down, the voltage should be above .850 with the flame on it, if the reaction time of the sensor gets too slow that causes problems. the other type of sensor needs to be tested on the car, you can play with the throttle and watch the readings

[talltorontoguy]

Thanks Winfred.

The codes on the car are saying that all 4 are bad. I can replace the front ones easily, but the rears I'm having a hard time finding replacements other than dealer.

I'll do the test but likely they are bad based on the codes.

Seems like one of those parts that's only available OEM.

R

[winfred]

unless you got something that poisoned them (like leaded gas or non sensor safe sealent) i doubt that all 4 died at the same time, how about the heater circuit? they getting power to that? that will piss them off and the car may not have a code for the 02 heater circuit, some cars have a seperate relay for it and others run it off of another relay, a 535 runs it from the fuel pump relay but most other bimmers have a stand alone heater relay


Thanks Winfred.

The codes on the car are saying that all 4 are bad. I can replace the front ones easily, but the rears I'm having a hard time finding replacements other than dealer.

I'll do the test but likely they are bad based on the codes.

Seems like one of those parts that's only available OEM.

R