I replaced the Thermostat today and afterwords I disconnected the negative battery terminal to reset the computer codes.
I let it sit for 20 minutes, reconnected the negative post and drove a couple miles.
The check engine light did not come back on.
I don't have a diagnostic scanner yet but I ordered one on eBay.
When the scanner code reader comes in the mail I will check it out.

There is a secondary DTC P0172 for bank 1.
"Fuel Trim System Rich Bank 1"

Have you not entertained the idea that your poppet injectors are leaking fuel yet? Or are we going to keep thinking the t-stat has something to do with it?

If the engine ecu was thinking the engine was running cooler than normal or seeing an ambient temp of 'its dicking cold out' I could see it trying to dump more fuel to bring engine to temp. This would NOT cause a rich condition on just ONE bank.

Yes I have entertained the idea that the poppet injectors are leaking fuel.
I ordered a code reader on eBay so I can check/erase codes myself and I plan ordering a fuel pressure test kit on eBay to test the fuel pressure myself.
Amazon of all places has a poppet injector kit that includes the fuel pressure regulator for $100.
The reviews are actually good on it.
I cleared the codes by disconnecting the battery for 15 minutes and the check engine light has been off since though.

Fuel rail pressure tester will not tell you if your poppet injectors are leaking.