Cranks but Will Not Start — Electrical Checks

Symptoms and context

• The engine turns over at normal speed but does not fire
• The engine occasionally almost starts or backfires
• No start after refuelling, after a repair or after the vehicle stood for a long time
• A security indicator or immobiliser message is present
• The fuel pump does not prime when the ignition is switched on
• A scanner shows no engine speed signal while cranking

Probable causes to verify

These are hypotheses ordered for checking — not a confirmed diagnosis. Prove or eliminate each one with the measurements below.

• No spark because of an ignition coil, module, crank or cam sensor fault
• No injector pulse because of a missing crank signal or an immobiliser block
• Fuel pump not receiving power because of a relay, fuse or control module
• Engine control module missing power or ground
• Blown fuse or failed relay in the engine management circuit
• Immobiliser or key recognition failure
• Corroded or damaged wiring at a sensor or actuator connector
• Camshaft and crankshaft signals out of time because of a mechanical fault

Tools and preparation

• A scan tool capable of reading engine data and codes
• A digital multimeter with DC voltage and resistance ranges
• A spark tester or an oscilloscope, used according to safe practice
• A noid light or injector test tool for checking injector pulse
• A fuel pressure gauge where the manufacturer provides a test point
• A wiring diagram for the engine management circuit

Step-by-step tests

1. Read codes and live data first Connect a scan tool and read stored and pending codes, then look at engine speed during cranking, coolant temperature, throttle position and immobiliser status. A missing engine speed signal while cranking is a strong clue to a crank sensor or wiring fault.
2. Confirm spark with a safe test Use the manufacturer-approved spark test method. A strong, consistent spark on the test device confirms the ignition side; no spark narrows the fault to the coil, ignition module, crank or cam sensor, or their wiring and supply.
3. Check injector pulse Use a noid light or the specified injector test tool. A pulse confirms that the control module is commanding fuel; no pulse on all injectors points to a missing crank signal or an immobiliser block, while no pulse on one injector points to that circuit.
4. Check fuel pump supply and operation Listen for the pump prime at key-on and measure voltage at the pump connector or the pump control module during cranking if accessible. Check the fuel pump fuse, relay and control circuit. A pump that receives power but does not run is a pump fault; no power points upstream.
5. Check module power and grounds Confirm that the engine control module has its supplies and grounds, using the wiring diagram. A module missing a ground can power up intermittently or not at all, producing a no-start with no useful codes.
6. Check the immobiliser and security status Confirm that the immobiliser recognises the key and that no security block is active. If the system is not satisfied, the module may deliberately disable spark or injection, and the fault is in the key, the reader or the security module rather than the engine management.
7. Verify the crank and cam signals Inspect the sensor connectors and wiring, and use a scope where available to confirm the signals while cranking. A signal that is absent, distorted or intermittent will prevent both spark and injection on many systems.
8. Repair and verify the complete start Repair the identified fault, clear the codes, and confirm a normal start and run. Recheck that no codes return and that the engine data is sensible, because a no-start repair that leaves a marginal signal will fail again.

Expected observations

• A strong, consistent spark should be present at the specified test point during cranking. The exact test method and spark energy are defined by the manufacturer and the ignition system, not by a universal value.
• Injector pulse should be present while cranking on a system that is enabled. The pulse width and timing depend on the engine management strategy and temperature, so the presence of a pulse is the first question and the exact width is secondary.
• The fuel pump should prime when the ignition is switched on and run during cranking, according to the system’s strategy. The specified pressure belongs to the vehicle and must be measured at the manufacturer’s test point.
• Engine speed should appear on the scan tool while cranking. If the value stays at zero, the control module is not receiving a crank signal and will usually not command spark or injection.
• Power and ground at the engine control module must be present and stable. The exact voltages and pin assignments come from the wiring diagram and the manufacturer’s specification.

Interpreting the result

• Spark and injector pulse present but no start shifts the diagnosis toward fuel delivery, compression or mechanical timing, even though the complaint is “electrical”.
• No spark with injector pulse present points to the ignition side; no injector pulse with spark present points to the fuel control side; neither present points to a shared signal such as the crank sensor or immobiliser.
• No engine speed on the scan tool while cranking is a strong indicator of a crank sensor, its wiring or its reluctor, though the module supply and ground must also be confirmed.
• A fuel pump that primes but does not run during cranking can indicate a control strategy, a relay fault or a pump that fails under load; check the supply and the command.
• Codes that point to an immobiliser or communication fault should be resolved before replacing ignition or fuel components.

Common errors

• Replacing ignition coils or fuel pumps before checking spark, injector pulse and fuel supply.
• Ignoring the scan tool’s engine speed value during cranking.
• Testing for spark near a fuel leak or with an unsafe method.
• Assuming a code always identifies the failed component instead of testing the circuit.
• Overlooking the immobiliser because the engine cranks normally.
• Cranking repeatedly until the starter or battery is damaged while the real fault is a missing signal.

Frequently asked questions