O2 sensors with stainless steel exhaust


Veteran Member
May 19, 2000
Los Angeles
Something I just learned after running stainless headers since the mid-90's.

Looks like you should run a 4-wire sensor.

Long story short, I had been getting significant fuel-trim variability at low MAP values for a long time that I couldn't really pinpoint the cause of. Because it was low-mid MAP values, the performance of the car wasn't significantly impacted, but it always kinda bugged me in terms of knowing the variability was there.

I recently found out that stainless steel evidently can have relatively poor electrical properties in terms of getting a reliable ground for O2 sensors that depend on the exhaust system for it's grounding.

The 3-wire sensors have a ground wire, but its for the heater element only. Whereas the 4-wire has a ground wire dedicated to the sensor element itself. On my old TPI computer, the O2 sensor ground in the harness is actually just a reference ground that is connected to chassis ground (since the 80's-90's O2 sensors were only 1-wire). So if the O2 sensor ground isn't exactly the same as chassis ground, it'll cause fuel trimming issues.

I bought an AFS-75 sensor and took the sensor element ground and tied it directly to the ground stud for the ECM.

At that point the fuel trim variability was eliminated. I had been getting BLM readings varying from 126 to 140 at times between 20 and 40 kPa. Now, the variabilty is only 126 to 130. What I notice is a bit smoother and more linear transition between closed throttle to moderate throttle (it's subtle, but noticeable).

I know it was this ground as the culprit since I've tried different 3-wire NB O2 sensors over the years and always got the same result.


Veteran Member
Apr 3, 2010
Using the headers for a ground, that means the only place for the headers to ground to the engine is with the header bolts/nuts/whatever, as the header gasket would otherwise insulate the header flange from the cyl head. THEN the engine grounds must be good and numerous (one at the rear of each cyl head to the body, plus good internals in the battery ground cable).

Good that you finally discovered the cause and fix of the issue. Factory EFI or otherwise?



Veteran Member
Aug 1, 1999
I got a piece of standard exhaust pipe, cut a section of it, welded a bung for the sensor, soldered a ground wire connection to it, cut a hole in the exhaust pipe/collector and clamped the half sleeve on it. And it's easy to remove and reuse as needed.