Diagnose

Cranks but will not start

Split by generation: HFM/LH cars want spark, fuel and a living harness. ME cars want a crank sensor that still works hot.

Check

Does it fire at all — a cough, a hint of running — or is it completely dead?

A cough is fuel + some spark. Completely dead is often immobiliser-unrelated on these cars (most R129s have no factory transponder) and points at crank/TN, OVP, or a harness.