2022 Kenworth w900L Headlamp Issue

I think the low beam light is a dual filament bulb if I'm remembering it right. One wire is low beam, other wire sends power when high beam is activated, 3rd wire ground.

Have you tried a new headlight? Some of these newer units with cab ECUs won't power up a circuit if it detects its open (ie unplugged or burned bulb).

 

Exactly, replace the headlight. "On dash code reads Chas. Node Pwr. Input Faulty check fuse and low beam lamp fault detected." That should work but if it does not then move on from there.

 

I'd have isolated the left low beam wire at the node and powered it up with a fused jumper lead to confirm the wiring was good and also checked that the headlight ground was good before throwing money at it. No idea if Paccar grounds the headlights to the frame or if ground comes back to the node though. Have to take that into account before sending power down circuits with computers in them.

 

The attached diagram shows the headlight ground goes to a stud on the inside of the frame rail (circled in red). Diagram not for exact same year. Then there are 3 harnesses that go from the node to the headlight. There was another thread on here recently (about March 11) where the left turn indicator had a problem.
Interesting that the message states "Input fault" but you say that all the inputs are good. Circuit needs to be under load, it can be ok when unloaded but not ok with a load. Which is why a good tech will use a bulb to test the circuit under load.