I recently replaced both upstream O2 sensors on a 2000 S500 and now it is back with upstream sensor issues. I'm getting a fault that indicates the resistance is not correct for the upstream sensors. Unfortunately, I was trying to save the owner some money and purchased the lowest priced ones I could find for the application. Has anyone ever seen this before?

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Jim

Only when the cats where bad. Its a peculiar situation. The upstream O2 being so bad that they killed the cats because the engine had adjusted so far or leaded fuel was used, etc.

Then with new upstreams the "proper" AF mixture is being used. The spend cats are no longer working within spec and the downstream O2s read this and throw a code.

Or, I suppose, the front O2s are the wrong ones and causing the cats to work overtime.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Downstream O2 reading are steady as a rock.

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Jim

According to a few websites the oem bosch units are 13642 for front units and 13782. They're about 95/each.

Bosch has universal (15738 front and 15738 rear)ones that you must splice into the harness they're about 20$ cheaper (77/each) than the oem ones. Same sensor just different wiring bits.

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82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Got the car back in with check engine light. Codes were:


Right O2 sensor, before TWC (KAT), short circuit to postitive/Resistance of sensor heater too low - P0125 - current and stored
Left O2 sensor, before TWC (KAT), short circuit to positive/Resistance of sensor heater too low - P0155 - current and stored

Right O2 sensor, before TWC (KAT), open circuit - P0134 - stored
Left O2 sensor, before TWC (KAT), open circuit - P0154 - stored

If it just says stored does that mean that the fault is no longer there? I guess current and stored means that it is a fault that came up and is still there. These O2 sensors came from AutoZone and were sold based on the model/year application (they were Bosch sensors - just did not capture the number on them). I was under the impression that an O2 sensor was not real complicated. Any thoughts/comments are appreciated.

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Jim