Gasoline engine cranks but will not start

Either a confirmed cause of the no-start, or a mechanically sound fuel, spark and compression result set that shifts the investigation to engine management inputs.

Confirm the battery is charged enough to crank the engine at normal speed. Note whether the immobiliser indicator behaves normally, and ask whether the fault appeared suddenly or gradually.

• petrol Not verified
  This vehicle applicability has not been confirmed against the manufacturer documentation for your exact vehicle.
  General method applies to port- and direct-injection gasoline engines. Verify the fuel pressure specification for the exact engine before measuring.

1. 1

   Battery discharged or failing, so cranking speed is too low

   high
2. 2

   Charging system fault that flattened the battery

   medium
3. 3

   No fuel delivery (pump, relay, pressure, empty tank, inertia switch)

   high
4. 4

   No ignition spark (coil, ignition relay, crank/cam signal)

   high
5. 5

   Immobiliser or anti-theft system inhibiting start

   medium
6. 6

   Mechanical failure (timing, compression) after the electrical checks pass

   low

1. 1

   Diagnostic

   Establish cranking quality

   Measure battery open-circuit voltage, then measure the voltage at the battery while a helper cranks the engine. Watch the rate at which the engine turns over.

   What did you find? A charged battery near its rated open-circuit voltage, and cranking voltage that does not collapse, with the engine turning at a normal rate.

   Measurement: V

   Authored outcomes

   • Fault Cranking is slow, or cranking voltage collapses
     Treat this as a battery, cable or earth fault first. Charging and starting circuits are covered by a dedicated procedure.
   • Normal Cranking is normal
     Move on to fuel delivery.
2. 2

   Diagnostic

   Battery and starting circuit check

   Charge and retest the battery. Inspect and, where necessary, clean the battery terminals, the engine earth strap and the starter feed. Repeat the cranking voltage measurement after any repair.

   What did you find? Cranking voltage recovers and the engine turns at a normal rate.

   Authored outcomes

   • Normal Cranking is now normal
     Continue with the fuel check.
   • Fault Cranking is still slow after a charged battery and clean connections
     The starter motor or its circuit is now the leading suspect. Confirm with a voltage drop test before replacing parts.
     End of this branch
3. 3

   Diagnostic

   Confirm fuel delivery

   Listen for the fuel pump prime at key-on. Check fuel pressure at the rail or with a suitable adapter, and confirm injector pulse with a noid light or a suitable current probe. Check the inertia/fuel cut-off switch where the vehicle has one.

   What did you find? Pump prime audible, fuel pressure within the general range for the system type, and injector pulse present while cranking.

   Measurement: bar

   Authored outcomes

   • Fault No pump prime and no pressure
     Investigate the fuel pump circuit: fuse, relay, inertia switch, then the pump. Confirm the relay is being commanded before condemning the pump.
     End of this branch
   • Caution Pressure present but no injector pulse
     The engine management system is not commanding injection. Move to the spark and synchronisation checks; a missing crank or cam signal explains both.
   • Normal Fuel delivery confirmed
     Continue with the ignition check.
4. 4

   Diagnostic

   Confirm ignition spark

   Check for spark at a plug or with an inline spark tester on each coil. Inspect the plugs for fuel wetting, electrode condition and gap. Where coil-on-plug is used, swap a known-good coil to a different cylinder and retest.

   What did you find? A strong, regular spark on every cylinder.

   Authored outcomes

   • Fault No spark on any cylinder
     Check the ignition relay, fuses and the crank/cam signal the module needs before spark can be commanded.
   • Caution Spark present and plugs are dry
     Spark with dry plugs points back to fuel delivery. Re-check pressure and injector pulse under cranking.
   • Normal Spark confirmed on all cylinders
     Move to the mechanical and synchronisation check.
5. 5

   Diagnostic

   Check engine synchronisation inputs

   Scan for stored and pending codes. Inspect the crank and cam sensor connectors and wiring. Where the tool supports it, compare commanded and actual cam/crank correlation.

   What did you find? No correlation or signal codes, and clean connectors.

   Authored outcomes

   • Fault A crank or cam signal fault is present
     Diagnose the signal circuit before replacing the sensor: a wiring or air-gap fault produces the same symptom.
     End of this branch
   • Normal Signals are clean
     Continue with the mechanical check.
6. 6

   Diagnostic

   Mechanical integrity check

   Run a relative compression test if the tool supports it, otherwise a compression or cylinder leak-down test. Check camshaft rotation and the timing belt or chain condition.

   What did you find? Even compression across cylinders with no timing fault.

   Measurement: bar

   Authored outcomes

   • Fault Compression is uneven or absent
     Stop the electrical diagnosis. This is a mechanical fault: timing, valve sealing or ring sealing.
     End of this branch
   • Inconclusive Compression is even
     Fuel, spark and compression are all present. Re-examine engine management inputs and immobiliser status with live data.
     End of this branch

• Cranking voltage stays above the general minimum while the engine turns
• Fuel pressure or injector pulse confirmed present
• Spark confirmed at the plug or with an inline tester
• Engine starts and idles without a stored misfire code
• No diagnostic trouble codes return after a road test

• Understanding the check engine light General educational guidance · AutoTools
• Fault code library: crankshaft and camshaft position codes Trusted technical reference · AutoTools OBD reference

  General code descriptions and typical causes, not a manufacturer procedure.