Run the heater on HIGH inside and see what kind of output you are getting. Also, this is a great way of keeping the engine from overheating if the water pump is still functioning properly as the heater core is not t-stat controlled.
Yes, first thing to do is check codes to start diagnosis.
Check your throttle body, clean it if necessary, press on it to see how freely it opens, release it to see if it returns back to almost closed position.
The pressure switch does not have a reference voltage, it is switched ground only.
Place 1 probe on metal grounded surface and other on part being tested. Set meter to Ω (ohm):
When testing the switch, you should have infinite resistance (no continuity) in Park, and 0 (no resistance) in Drive...
The workshop manual lists only the oil pressure switch circuit being the cause of P0841, either the switch, the harness, the wiring to the TCM, or the TCM itself. If you have instructions that list otherwise, I'm sorry I can't help with that.
Did you perform the harness inspection?
On-vehicle Inspection (harness inspection)
1. Remove the battery duct and battery cover.
2. Disconnect the negative battery cable.
3. Remove the under cover.
4. Disconnect the cord assembly connector.
5. Verify that there is no continuity between the cord assembly terminal and ground.
Upon further reading, the workshop manual has conflicting data regarding the MIL/AT lights.
I don't have personal experience with P0841, so I can't speak to the causality except what the workshop manual states. It is a NO switch that provides ground to TCM when pressure is applied and can...
I'm glad you got it repaired for a reasonable cost. What you are referring to is the EPC, pressure control solenoid.