New to this forum but long time Ex owner. This is my second 2000, 4wd, Limited, 6.8 V-10.

Just bought this one last month and after two weeks of local driving the following problem showed up:

1.Week of July 5 through 11: With vehicle cold (65*) started and placed in reverse to back out of garage. After slight movement engine stalled like a locked up torque converter or turning ignition off. CEL illuminated. Turned off ignition and restarted. Again in reverse had same stall. Third time no problem but CEL still on. Code is P0720 using "Actron Auto Scanner."
2.Erased Code P0720 (OSS circuit malfunction) and P0307 (Cyinder 7 Misfire Detected). Drove car approx 20 miles without incident. No stumbling, no vibration, no detectable misfire, shifts okay, speedometer okay.
3.Next day with vehicle cold, started to back out of garage and stalled with CEL illuminated and O/D light in shifter blinking. Recycled ignition, started and no O/D light but CEL still illuminated. Vehicle did not stall. Checked code and P0720 set.
4.Replaced OSS on tail shaft of transmission. Same stalling problem with P0720 set.
5.Replaced #7 COP w/aftermarket type to test possibility of bad COP because of earlier P0307 DTC. Same problem exists, first start of the day the vehicle will stall as soon as it starts to move in reverse.
6.Replaced rear ABS sensor. While doing so noticed the ABS sensor wire harness was rubbing the tailpipe and the split, wire conduit was partially melted. Inspected wires (2) within and both appeared okay. Safety wired the harness away from the tailpipe.
7.Let the vehicle sit awhile. Outside Air Temp 84*. Started, placed in reverse, moved a couple of feet and engine stalled and O/D light on steady. Recycled ignition and started again, backed out with no stall or warning lights. No CEL on. Drove 10 miles without incident.
8.Allowed vehicle to cool, started and attempted to back out of garage. Engine stalled, CEL illuminated and O/D light flashing, code P0720.
9.Sunday morning 12 July: 71*, Allowed vehicle to idle in P in garage for 10 minutes before trying to back out. Vehicle did not stall. CEL still on from previous day, no O/D light. Erased P0720 code. Vehicle operating normally.
10.Sunday Noon 12 July: 85*, Started, placed in reverse, backed out and no stall or CEL or O/D light.

Discussion:

After considerable research online at ford-trucks.com found P0720 is a common problem but not easily diagnosed. In most cases the gremlin is COPs and or deteriorated plug boots but in this situation the engine appears to run smoothly and the failure/errors only show up when the vehicle is moved in reverse gear after a cold startup. Once it stalls then restarted it runs fine.

Made a quick check of alternator for failed diode using DVM since I don't have a scanner. Checked okay for voltage and AC ripple.

I do have an appointment on Wednesday with my local auto repair shop for troubleshooting. Will have them do continuity check between OSS and PCM and an AC ripple test.

Does this sound like an electrical circuit problem or internal transmission problem? Want to make myself as smart as possible when I talk with the techs. Thanks!

 

AC/DC WELCOME again Sorry you got so little trouble free time on this one Mark is our resident TRANSMISSON EXpert so I'd get TRANSMISION PROBLEM ? into the title. He will chime in when he sees it


Just a thought : BACK in into the garage tonight & in the Morning burn it out like it's a rental Just so's you will know if it's limited to reverse.

 

Harley4jcs, bet I am a more ELDER user than thee! Thanks for the hint about changing the title.

 

I've never seen a P0720 be an internal transmission problem. It's highly unlikely.

It is either a bad sensor or a damaged/corroded wire between the PCM and the OSS.

 

Thank you for your assessment, Mark. That it is not likely to be an internal transmission problem is indeed encouraging and it is great to have people with your knowledge and experience on this forum.

As an update, I followed harley4jcs' advice and backed into the garage last night. This morning, I started up and drove out without a malfunction and then backed in again, still no malfunction. So I guess this can be classified as an intermittent problem. Not holding my breath but I will drive it for awhile and see what happens.

 

delete, wrong post.

 

I chased a P0720 code for WEEKS in my long-dead 2001 7.3. New OSS, read out the harness, consulted with Mark heavily as well.

It ended up being the freaking driver side battery, swollen on the bottom (tray side), and thus not immediately obvious. I want to say in my research back then, a weak or sporadically so, alternator can also cause some electrical gremlins to manifest.

I’m not around here much anymore, but that’s also the extent of my immediate memory on the subject. Happy hunting and good luck, electrical issues suck.


Edit: Also, to any of the OG’s tag may have seen my reply - something unrelated brought me back briefly, I’m not back for good. Yet.

edit 2: I didn’t realize this was an ancient thread revived from the ashes accidentally by someone else. I’ll see myself out 😂