1993 Ford Ranger stuck lean Bank 1 Code 172

Hey I am new to the forum but not to Scanner Danner techniques.The info provided in your books, videos ect are some of the best to learn how systems work and how to properly diagnose them. That being said I have a 1993 Ford Ranger 3.0L in my shop that i am stuck on. Truck runs fine but has a lean bank 1 (code 172 for 02 sensor lean) --i gave it propane through air cleaner and Bank 2 goes full rich about 980mv but Bank 1 doesn't respond. Unplugged B1S1 and used positive post of battery to signal wire with fingers and voltage will never go past .500mv on the scan tool - I tried test light just to verify and got same result. I next soldered in a new signal wire in from pcm to b1s1 o2 and there was no change in reading on scan tool (ranges from .002 to .035 no matter at idle or 2000 rpm) Repeated test light test and same result. Next i went to pcm and to check grounds for resistance -- but i couldn't get a good diagram on 02 sensor how it is grounded (or i think ford calls it signal return) so i cleaned up three main ground connection on left fender and one in front of battery on radiator support --no change. Since bad ground will cause rich condition not lean so i wasn't to concerned with a ground problem but to be safe I put in temporary ground directly to o2 senor bank 1 but no change. I found local salvage yard that had right computer for $50 (sorry i hated to do it but i have so much time into this and couldnt find any other problem) Switched in computer and exactly the same issue.... i am not sure where to go next. Any diagram on this o2 senor circuit would really help. I have ALDATA but diagram is incomplete on 02 circuit. Could it be i just happened to get second bad pcm with same problem or am i missing a ground for computer? ANY HELP WOULD BE GREATLY APPRECIATED! Jeff Seeger (Seeger Repair Shop) (641) 278-0425

Eh, don't sweat the PCM. For $50, I think we'd all take that gamble! :lol:

If it makes you feel better, I can't really find anything wrong with your testing. You should absolutely be seeing higher than 500mV on your scanner. Just to make absolutely certain, you ran the new wire from the grey/light blue wire at the O2 to pin 44 at the PCM?



The other thing that occurs to me is a scanner malfunction? As in, for whatever reason, the scanner itself won't display higher than 500mV for that PID. Even though it should. Perhaps try another scanner? If you haven't already.

Hey Tyler i got on the forum to post the fix for this truck and saw your reply. Thanks for taking the time to post the diagram on this 02 sensor. Here is process that figure this thing out. Got a diagram from another shop with identifix and it had missing page i needed and right away i noticed that the signal return splices into the scan tool dlc and that pointed me in the direction of my scanner (great thought in your reply to check that - you nailed it!). I disconnected the scanner (Actron CP9690) and hooked up my dvom to the sensor signal wire and wouldn't you know the o2 is ranging perfectly! Now here is the interesting part is i left dvom on signal wire and plugged scanner in again and as soon as it powered up the o2 went lean again - dvom and scanner both read the same. So scanner actually was pulling voltage down to less than 1/2 a volt and scanner had to powered up to actually pull down signal voltage - it WOULDN'T cause the problem just plugged into the dlc connector. Defiantly will not forget this lesson! Problems like this really help you understand how a system works. THANKS AGAIN for the input. Hope this will help someone else in the future.

:lol: Good timing! Sorry I didn't get back to you earlier. Thanks for updating with a fix.

You're right, the signal return is totally tied into the DLC. Found it on a second look at the diagram. Kinda angry I didn't see it earlier. What I'm still not sure about is, why would an issue on the signal return cause a low voltage problem on the signal side? That signal return circuit is tied into both O2's, so any issue with that circuit should have changed the readings on both sensors. I'm not debating your findings, just trying to figure the cause/effect.

FWIW, I went through a VERY similar problem with my Modis Ultra a couple years ago:

www.scannerdanner.com/forum/off-topic-se...with-video.html#3302

So, if the scanner was part of the problem, then what do you think was the original problem? :huh: