Thanks for the help.

The fluid and transmission dip stick are super clean. My guess is that prior to purchasing the car the transmission fluid was changed and/or the transmission was rebuilt/replaced. The engine also had markings indicating it had been replaced ('"A" good' was written on it).

Prior to realizing that my gaskets were going out, I thought it might be the PCM. So, I tried another PCM, and I got the same result. So, I'm thinking that the PCM harness may be the problem... except that it was about a month ago that I messed with the PCM harness, and I don't know for sure if the code came up then or more recently.

Lastly, I read it was a good idea to check the "20-way connector" from what I read here:
DTC P1810 TFP Valve Position Switch Circuit 3

So, I'm thinking it's harness/wiring related. I'll try the PCM harness first. If that doesn't do it, then I'll see if maybe I bumped one of the transmission harnesses when I did the gasket changes. Which one is the 20-way connector?