I had to have my transmission rebuilt about a month ago. Second time in three years. The guy who rebuilt it last time assured me that everything was new in the case, and all updates had been performed. When it left me on the side of the road a while back, the transmission shop said that there was no way everything he was looking at was new.... so now this guy tells me everything is new this time around...

So a couple of days ago, I hit the interstate and my truck wouldn't shift past second gear. I pulled over and checked everything out. All looked good so I hit the road to try to make it home, and everything worked fine. Called the trans shop and had it towed to him.

He calls me today and says that the transmission is fine. He says that I have codes P0300 and P0856. He says that the engine is missing and the computer is keeping the transmission from going past second gear. Anyone ever heard of anything like that? I never have, and I searched the forum and found nothing. Figure I'll bring it home and start going through ignition stuff to see if I can find a miss. Thanks.

 

Never heard of either of those situations linked to any trans malfunction.

For the P0300 (random mis) when were the plugs last changed and what plugs were used?

The P0856 relates to the Electronic Brake Control Module, either it, or the harness or connection to the harness is faulty or has a short...... or...... there is a problem with your PCM.

If I remember correctly, you have either an 06 or 07, so your truck has no separate Transmission Control Module and instead the Trans is controlled by the PCM. So, a faulty PCM could cause the Trans fart.

Disconnect the battery, wait at least 30 seconds, more better (good time for a beer break), and reconnect it ground first. That may clear a brain fart in the PCM if that's all it was. It obviously won't fix a bad PCM.

See what happens after that.

 

Thanks Doc. You're right. It's an 07. I had never heard of that either. I'll try what you said. To answer your question, I installed new plugs, coils, sensors, and PCM just a couple of years ago. Plugs are Delco Iridium. All parts are Delco. I'll check them all. They shouldn't have gone bad already, but stranger things have happened. Thanks!