My 2003 S40 won't start. Battery and starter are fine, cranks vigorously when the starter solenoid is jumpered. I replaced the neutral safety switch no success, then noticed none of the dashboard warning lights are coming on when the key is in run position, code reader reads error (no communication). Replaced the key start switch, no change. All fuses and the engine system relay check ok. I am thinking the Central Electronic Module pn 30621304 is the culprit. Any ideas?

 

It's not your CEM. Yes they have problems - but with headlamps not working. The early sv40 CEMs are not like other Volvos and control very little. And stop replacing random parts without a diagnosis - as you have found out - it gets very frustrating and possibly expensive.

Your car has an immobilizer. Have you checked fuse B11? A11 and 14? Note B11 also powers the instrument cluster. The immobilizer grounds the starter relay - is the starter relay ok? (swap with same part number if possible to test)

 

Thanks for the reply and for halting my random parts replacement program. 😊Checked all the fuses in the interior and under the hood already, but went back and
re checked the fuses you identified, all good. Tried swapping the start relay with the one beside it (same part #) no effect. All lights work, door locks and accessories work with key in run position but check engine light is not illuminating on the dashboard, car won't start or shift out of park. Thanks for the wiring diagram, gotta scratch my head over that for a bit.

 

To be more explicit, none of the dashboard warning lights come on ( batt, coolant, p brake, oil, immobilizer etc.). I was thinking if it was just an immobilizer problem I would see that warning appear. I haven't changed keys or anything like that, but I have had the battery in and out a few times.

 

Found none of the engine run circuits were energized with the ignition in the run position. This led me back to the ignition switch that I had already replaced...BUT my car has an aftermarket extra relay connected between the ignition switch and the main wiring harness. The extra relay bench tested ok but I measured the ground wire resistance to chassis and found an open circuit. Whoever installed it ran the ground wire to a retaining bolt on a plastic relay box and it wasn't making a good connection. Ran the ground to a steel body part and now all back to normal. I think that poor ground was responsible for my intermittent no start, until it finally disconnected permanently.
The extra relay doesn't show on the circuit diagrams I have seen but it looks like an official modification with it's own wiring harness and proper connectors.

I am guessing it is there to save the ignition switch from carrying higher currents and burning out.