jimsRX300

What is this P0778? It's critical to vehicle operation.

I cleaned the transmission speed sensors and replaced bank 1 sensor 1 O2 sensor. Now, something got unplugged. What? This thing is dead without it.

I can't seem to figure it out. Some cannon plug rear of the intake manifold?

timmui

A quick search for DTC P0778 returns this:
I know this means that one of the transmission solenoids is an issue (or the wiring to the ECM) but I wouldn't know where to look for the solenoid itself. It is possible that there is a control module for the transmission that may be contributing to the issue.

Also, I saw you did an engine swap recently. Is it possible some of the ground wires aren't connected? You may want to check if some of the harnesses are loose as well. GL!

jimsRX300

After I swapped the engine, I had transmission stuttering between 1-2 and 2-3. I did not use OEM fluid. I also had P0135 DTC.

Purchased new Denso O2 sensor from Amazon and replaced it.

Cleaned both transmission speed sensors and replaced fluid with Toyota T-IV. Go for a test drive, transmission is hard shifting bad, P0778. I think I messed up a wire on top of the transmission. I'm gonna dig around tomorrow AM. I bet it's a loose wire.

jimsRX300

Yuuup! Ground wire! Actually, I'm amazed it even started to begin with.

All ground wires cleaned. No CEL and shifting corrected.

salimshah

Which ground point was loose? Does not mean every one would have the same gnd problem, but its good to build statistics.

Since you went through ALL, I would like to know where each exists.

Salim

2KHarrier

Agreed. However I'm glad it was an easy fix for you.

jimsRX300

Interesting enough, the 300 should not have even run after the swap. Not only was every chassis ground super corroded, I forgot to attach the bank 1 (left side, or rear) grounding strap completely.

I'm certain the P0778 is just a generic OBD code when the ECM can't figure out a ground or there is a short in the system. I used a wire brush and emery cloth to clean both sides of all engine grounds and the battery ground.

When I changed the O2 sensor, I must have moved that unattached ground.

jimsRX300

I found the most corrosion on the two 10mm fender well ground bolts.

The second most was the negative battery chassis ground found underneath the battery tray.

Then I found the rear engine harness not connected.

jimbug

Congrats!! Hoping for nothing but good reports going forward. Thanks for all the details. It helps build our knowledge base here.