How do I diagnose a crank but no start?
Short answer: Confirm the basics of fuel, spark, compression and timing in order, using scan data for crank sensor rpm, security status and fuel pressure before replacing parts.
A crank but no start means the starter works, so focus on what the engine needs to run. First, use a scan tool to check codes, engine rpm while cranking, and immobilizer status. Zero rpm while cranking points to the crank sensor or its circuit. A security light or theft code points to the immobilizer.
Next, check fuel. Command the fuel pump on or measure fuel pressure with a gauge. On injected engines, check injector pulse with a noid light or scope. Then check spark with a spark tester. If fuel and spark are present, check compression and timing; a jumped timing chain or broken belt causes low compression on all cylinders.
Battery voltage while cranking matters too, since low voltage can stop modules from operating. Wear eye protection near fuel, keep ignition sources away from any fuel leak, and disable the fuel system before compression testing.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.