How can I verify the timing is correct and what sensors are on the 5 Volt reference circuit A

1998 Grand Prix SE 3.1L with an “M” in the VIN

The car died on the highway shortly following an oil change and radiator flush. The driver said the check engine light came on and a few miles later the engine made a clunk and died. She had it towed to a nearby garage and they generously offered to rebuild the engine for about $3200.00 (about 6X the value of the car) but the mechanic wouldn’t say what was wrong.(Rip off.).
We couldn’t get it started so I picked the car up and towed it (200 mi.) On a dolly.
While cranking the engine it would initially fire a few times and then sounded like it was alternately trying to fire three times then nothing then three times and nothing... In a sort of burmp burmp burmp... Whirr... And so on as long as I turned it over. Then if I tried to crank it again it wouldn’t fire initially but did the same burmp burmp burmp... Whirr... Burmp burmp burmp... Whirr.
The DTC set 1 code: P1635 “5volt reference circuit A” I checked all the sensors I could find for the 5 volt signal but I am not sure if I got all of them. Every one I checked was good (5v.).

So far here is what I’ve checked:
Fuel pressure 40 psi.
Coil packs primary and secondary passed
ICM Passed

Wiring harnesses looked ok and are properly seated.
I have spark so I ruled out the crank sensor.

At this point it sounds to me like it jumped time or possibly a broken timing chain

If the problem is with the timing or a broken chain wouldn’t that set some other codes?
Is there any quick way to check the timing without tearing it down?

And what systems use the 5v. Ref. A circuit?