Engine Cranks But Won't Start: An Evidence-Driven Diagnostic Framework

Symptom-first framework

This is not a "common causes" list. It is an evidence-driven framework that uses the symptom to narrow the system, then tests to confirm.

1. Clarify complaint — cranks but will not fire? Fires then stalls? Intermittent?
2. Confirm operating condition — cold/hot, wet/dry, after a fill, recent work
3. Retrieve DTCs — all modules, not just the engine
4. Inspect freeze-frame / live data — the conditions when it last failed
5. Identify affected system — crank, immobiliser, fuel, ignition, mechanical
6. Form differential — rank hypotheses by the evidence
7. Select decisive test — the test that most changes probability
8. Update hypothesis — from the result
9. Confirm cause — strong, confirmatory evidence
10. Repair
11. Verify the original symptom — under the condition that failed

Split 1 — Battery / cranking

Does the starter crank strongly?

• Weak/slow crank → battery, terminals, grounds, starter circuit. Check resting voltage, cranking voltage, voltage drop. A weak crank often masquerades as a fuel/ignition fault because the ECM cannot operate at low voltage. See the Battery Voltage guide.

A strong crank moves the investigation on.

Split 2 — Immobiliser authorisation

Many ECMs will not fire injectors or coils without start authorisation. On BMW, that is the CAS/EWS path; on others, a similar immobiliser.

• Immobiliser code present? → authorisation, not fuel/ignition
• Key recognised? → CAS/key/alignment
• Terminal-15 / wake-up to the ECM? → power distribution

Confirm the authorisation path before testing fuel or ignition. See the ISTA / BMW hub guides for the CAS path.

Split 3 — Engine speed input

The ECM needs a crankshaft signal to trigger injection and ignition.

• Tachometer moves during cranking? → No: likely no crank signal (P0335). Scope the crank sensor.
• Crank signal clean? → See the Crank Sensor Waveform guide.

Split 4 — Fuel supply

• LP supply to the HPFP adequate? (lift pump, filter)
• Rail pressure builds to the minimum that enables injection? → See the Fuel Rail Pressure guide and P0087.
• A no-start with low cranking rail pressure is often LP supply, not the HPFP.

Split 5 — Rail / fuel pressure

• Requested vs actual rail pressure during cranking — actual must rise to the enable threshold. See P0087 and P0191.
• Pressure falls after shutdown? → injector leak-down or check valve → hard start after rest.

Split 6 — Ignition (where applicable)

• Spark present during cranking? (petrol)
• Coil trigger command from the ECM? (scope or test light)
• No spark + no injector pulse → upstream (crank signal, immobiliser, ECM power)

Split 7 — Injector command

• Injector pulse during cranking? (noid light or scope)
• No pulse + no spark → crank signal / immobiliser / ECM
• Pulse present + no start → fuel pressure / injector flow

Split 8 — Compression

• Compression test — low compression across cylinders → timing or widespread wear
• Relative compression (scan-tool cranking-current test) — quick screen

Split 9 — Timing

• Crank/cam correlation (P0016/P0017) — a jumped chain shifts cam phase; scope the correlation. See the Crank/Cam Correlation guide.
• Belt/chain condition — interference engines risk piston-to-valve contact; do not crank repeatedly with correlation codes.

Split 10 — Air path

• Severe intake restriction (blocked filter, stuck throttle) — enough to prevent start
• Exhaust restriction — blocked cat/DPF can prevent start

Using the framework

Each split is a test that changes probability. Do not jump to a system without evidence. The most common no-start mistakes — replacing the fuel pump without checking the crank signal, or replacing coils without checking the immobiliser — come from skipping splits.

Links

• P0335 / P0340 — crank/cam signal faults
• P0087 / P0191 — rail pressure faults
• Fuel Rail Pressure — requested vs actual
• Battery Voltage — cranking voltage
• Crank/Cam Correlation — mechanical timing
• ISTA / BMW hub — CAS/immobiliser path

Common mistakes

• Replacing the fuel pump without checking the crank signal
• Replacing coils without checking the immobiliser
• Not reading all modules (immobiliser codes live outside the ECM)
• Condemning the HPFP without confirming LP supply
• Cranking repeatedly on an interference engine with correlation codes
• Not checking battery/terminals first on a slow crank