Can't read CEL

[fixkick]

make sure the colors match this firs.t


blue-yell  jumps to black, black is just ground.
only this wire color is correct for diagnostic mode , to see flash codes, DTCs/
freeze is timing advance spark freeze jumper input.
ISC is for setting ISC duty cycle to 50% (tricky this)
but the DIAG pin is super simple and always works if CEL lamp not burned out. or ECU bad.

[fixkick]

the ECU has 3 modes, normal
and failsafe (limphome mode) or as jeep calls it LIMP , when 1 sensor is dead bingo and LIMP you go, the ECU then emulates the bad sensor so you drive to a shop.

and last BACKUP mode. (never be here, MPG can be 10mpg) horrid MPG here. most times.\
The FSM book mentions it only in the most brief way. (super brief but, is non the less real)

The ECU has this very much secret mode (seen it myself) that is when for example it can not do limp mode.
for many reasons but for example, let say 2 or 3 sensors goes dead. and due to that gross state, the ECU can not emulate the 3 dead sensors!, and goes to BACKUP mode. CEL DEAD.

one more way is if the ECU main CPU chip can not process correctly (like noise on DC power bus on the ECU board) to BACKUP you go. (bad caps #1 cause here)
on some ECU the BACKUP is a second very low grade CPU chip (read dumb) and this backup CHIP uses just he TPS to measure air flow and inject on this very crude input.
I call BACKUP mode , lawn mower mode with its $3 carburetor , all nasty and running, black smoke out the exhaust.

the car usually runs if the CMP sensor works, the others can fail and run, nasty smog, and wasted fuel but runs. This is the magic of EFI..
if the CEL is stuck on and will not flash on demand by you (jumper) and nag switch is missing (fed cars) the ECU is doing that stuck CEL, for sure.
the ECU is messed up.( call it confused)
on many cars the CEL lamp is missing to hide the stuck on CEL to new buyer. common this.
but yours is not that. it goes glow.

BACKUP mode only happens in the worst cases of all, most times a good ECU will role out a change if many DTC errors key on if you remove al sensors.
like most of them and repeats endlessly , in loop , with the diag jumper inserted, just key on
I can do that even with a benched ECU , power , connected, cell lamp and PIN B above jumper ground, it goes wild with endless long DTC codes.

I hope this makes it easier to understand the ECU.