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I have a 2010 chevy cobalt automatic with trouble codes P0700, P0842 & P1810. Im getting hard shifting in all gears but sometimes shifts ok. It also seems to get confused on which gear to be in and doesn't always downshift. I replaced the transmission with a low miles used one and getting the same codes and same symptoms. Also, the cruise control will sporadically quit working and the door locks will not unlock when shifting to park. not sure if its related. I also tried manually grounding the tcm and checking fuses. any help is much appreciated!

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P0700 Transmission Control System Malfunction
P0842 Transmission Fluid Pressure (TFP) Sensor Circuit Low Voltage TCM
P1810 Transmission Fluid Pressure (TFP) Position Switch Circuit TCM


Try re-seating the 2 blue/gray connectors on the BCM also it appears your PRN switch might be out of alignment. Check fluid levels as per the procedure.
 

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Yes, I forgot to mention that I did try that as well, I also did this on all the modules under the hood on the drivers side
 

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Did you replace the fuse #25 even if it looks good? Did you re-use the fluid pressure sensor from the old transmission?
 

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Did you replace the fuse #25 even if it looks good? Did you re-use the fluid pressure sensor from the old transmission?
I didn't reuse any parts from the old transmission. I suppose it's possible I got another bad trans with the same exact problem, hard to believe that'd be the case though.. I didn't replace the fuse, but I will give it a shot tonight
 

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Did you re-align your park-reverse-neutral switch? That's why your doors are not unlocking. BCM doesn't know you're in park.

The GMLan communication doesn't go directly from ECM to TCM it passes through the BCM module first, fuse #25 sometimes corrodes and eventhough it measures good no signal flows through.
 

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I replaced the fuse and no change.. Another thing I should mention is the hardshifting comes and goes and so does the check engine light.. this morning it shifted ok, but now its back to hardshifting.. I also tried sticking a TCM from another cobalt with the same credentials and nothing changed, same codes.. I know these are suppose to be programmed by GM (at least new ones??) but you'd thing it would put out different codes if that was the issue??
 

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I ended up taking this to GM to diagnose, they pulled an additional code P0961 and I spoke to the tech and he was favoring it being an internal trans problem, but wouldn't know for sure unless he pulled the side cover (4hr job) I suppose it's possible, but just seems so unlikely that I get a replacement transmission with the identical problem. So instead of paying gm to do that, I might as well get a free replacement as it came with a warranty...I just don't want to install another trans just to have the same issue remain
 

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Check this out, seems to be a common issue and as told in the video others have replaced transmissions and it wasn't the issue. Wires heating up around the oil filter housing. I didn't find a bare wire on mine but the protective wire tubing had become hot from being up against the oil filter housing and broke apart and the wires were directly touching the oil filter housing and I rerouted the bundle of wires and so far so good. Even have my front speakers working again although I don't know if that is coincidence yet or not.

This is my first post so I hope I did the URL link correctly.

Edit: I'd also like to add that my Cobalt had the same symptoms as yours. Run fine for a few days and then after about 15 minutes of driving It would start hard shifting and that would happen for a couple days and rinse and repeat. I would also get the clicking sound from the P/N switch when braking with it not knowing that the car was in gear.
 
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