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2003 Audi RS6. After dinner yesterday evening, car cranks but on start.
Currently being towed back here.

Looking for suggestions where to start with troubleshooting:
- Vagcom scan for codes, write down, erase
- crank. See what codes come back
- crank position sensor bad IIRC creates 'crank no start'? checking crank sensor with Vagcom, engine module, measuring block where RPM show. Crank. I values (100 rpm or so) show the crank sensor good?
- likely unrelated: had very cold temp start problems. then: fired/started after a few tries, but did not want to take gas; idle slowly increased and stabilized, then driving fine; restarting fine when warm
- check fuses (which circuit/fuse can create 'crank, no start' condition?
- remove rear seat bench and listen for fuel pumps to kick in. I would assume car would at least fire a bit if only one of the 2 pumps (in tank and in line) died, or?
- remove spark plug and check for spark
- not sure how to test for fuel, except lots of cranking and taking plug out and check if wet, and gas smell from exhaust? Suggestions

Other notorious failure source that could lead to crank/no start?

Think you pretty much covered your first ports of call...

Get those all checked first and go from there.

As for checking for fuel/gas easiest way is to remove one of the fuel lines running into the rail (im not 100% sure on the C5 if it has High pressure pumps or not) but on the C6 you can just remove the fuel line going to the left or right HPFP to check for a steady stream of fuel/gas.

If you cant hear the pump prime its either the main relay or a bad pump.

Code scan first and go from there, your also correct about the CPS sensor... had simiar issue on my old saab, as long as it reads 100ish rpm on crank then the sensor "should" be ok.

Thanks for the suggestions. Car tow finally arrived (took 14 hours from first call to arrive for 40 mile tow; not a good day (10" snow) to ask for a tow; yet untold tickets (car was stranded in metered area). In heated garage since midnight.
Work will start in a couple of hours.

Crank sensor change on the RS6 4.2 appears to be a nightmare (for first reading of google posts) unlike on 2.7T (done a few there).

Might want to try this, does not cost anything

Not sure if you configuration is the same layout as the B7 RS4, but tried this on my neighbours Mk 6 golf

Had this last year, car would crank but no start, nothing showed on VCD's that was an out and out fault

Unplug the Maf sensor, tried again started 1st time, changed Maf no probs since

My neighbour was cranking his Golf, but wouldn't fire up, and I knew he was up early for a job interview, so got out of bed 5am, it was freezing, popped the bonnet disconnected Maf ,, bam on the button, plugged it back in and he drove off. 3 months later he is still running on the same Maf,

Worth a try

Conclusion:

Vagcom showed code below on pretty much all injectors/cylinders.
Erased codes.
Cranked (nothing).
Codes come right back.

Search for injector fuse. None found.
Checked again that no fuses are blown. None found.
Went down the list of relevant models/circuits/fuse.

Checking in fuse in position 28 (fuel pump). Slot is empty. Hmh. Figures RS6 with its two pumps has a different circuit/configuration (may separate, larger fuse or relay somewhere).

As always: can't hurt, stick a fuse in see what happens.

Car starts and runs fine, no codes.

Assumption: fuse was loose, last time door closing, knocked it out.

Have not bothered to take kick panel off to search for the fuse.

So next time: check fuses means check for blown fuses AND if check if all fuses are there.

Thats mental... good find!