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I had my trans rebuilt about 10k ago so just out of warranty. Drove it about 200 miles on the interstate yesterday no issues. Today I go into town stop and go driving for about 30 mins and on the way back to the house I stop at a red-light and notice my truck is surging a bit. I look down and notice my overdrive light is flashing. So I turn on forscan and see my transmission is running about 192 and I have a p0781 code. Normally my trans temp tops out around 175. I parked it at the house cleared the code and let it cool off. Drove it this evening and so far it has not come back. Seems to shift normal fluid still looks good, but I did notice my trans temp and eot temp and almost the same. Normally I see about 25-35 degrees lower on my trans temp. Any idea where I should start I'm thinking my stock cooler could be clogged or the bypass is stuck open.
So the trans is less than 20 degrees warmer than you've noticed before. Big deal. It could mean you stopped at a few more red lights than normal. Your cooler isn't clogged, nor is your bypass stuck open.
What you should be concerned about is the P0781 1-2 shift error code. That means that several 1-2 shifts didn't happen in the time that they should happen. You might want to use Forscan to data log some 1-2 shifts. Look at engine speed, turbine speed, output speed, and gear commanded.