2000 SOHC - P0141 - Need advanced help
So, I'm not a pro, but I'm not a greenhorn either. Just got a "deal" on a 2000 SOHC - had low compression on #4, but was not the valve seat drop. A reman head and I'm g2g. Or so I thought.
It is throwing a P0141 within a minute or so of engine start. I've been over the car with a fine-toothed comb, that's why I say I need 'advanced' help.
The story so far...
Car has new pre and post sensors, 5k miles ago with the timing belt change. With the rebuild I also did new coil, wires and plugs. Runs great, scan tool shows fuel trims normal. Fuel pressure normal, vacuum all good, intake is spotless (that took forever), overall everything checks out just fine.
When I had the header off, I could see down into the cat. I know you can't tell visually, but it looked unmolested.
P0141 is for the post-cat sensor, heater circuit. Just to be sure, I put on a new sensor. Tsted the front, plus new and old rear sensor - all read 3800 ohms across the heater, which (I believe) is correct.
Looking at the wiring diagram, on both connectors the grounds read good, and the input to the heater is reading nominal voltage.
So, I'm confused. I believe the sensors are good. The heaters are getting voltage. However, when I run the car with the scan tool connected, I see the front sensor start to show activity immediately, where the rear sensor seems to take a few minutes (ostensibly after it warms up from the exhaust heat.)
Everything seems OK, but it clearly isn't. Wondering what else could result in a 0141.