Fault Tree: Cranks But Won’t Start

Quick triage & tools

Scan first: read codes (all modules if you can) + live data that matches the symptom.
Power basics: battery voltage (resting + while cranking) and quick fuse checks before deep dives.
Have ready: basic scan tool, multimeter, and a way to confirm the symptom (road test / idle test).

Tip: if a step says “check wiring”, use Wiring diagrams basics → and Voltage drop testing →

What you need (minimal)

Basic scan tool that can read codes + live data (RPM, coolant temp, fuel trims if it starts).
Multimeter (battery voltage + simple checks).
(Optional) Spark tester / spare plug, fuel pressure gauge, noid light.

Related deep-dive: Crank / No‑Start Triage →

Decision flow

Confirm it truly cranks (starter turns engine at normal speed).
If it’s slow or clicks: treat as a no‑crank power/ground/starter issue first.
Battery voltage while cranking
- < 9.6V while cranking → battery weak, high resistance, or starter drawing too much.
- 9.6–10.5V+ → proceed.
Use: Voltage Drop Testing →
Scan for codes + check live data RPM while cranking
- RPM stays at 0 → ECU isn’t seeing crank signal (CKP sensor, wiring, gap/trigger wheel, power/ground). Start here.
- RPM shows ~150–300 → crank signal likely present. Continue.
Use: Crank/Cam signal checks (no scope) →
Immobiliser / key authorisation
- Any security light flashing / “immobiliser active” message?
- Codes like P0513 / B-codes / U-codes about key/cluster/ECU comms?
If immobiliser is active, you can crank forever with no start. Try spare key, check battery in fob (if applicable), scan body/immobiliser modules if possible.
Fuel delivery: do you have fuel pressure?
- No pressure / no pump prime → check fuses/relay, pump power/ground, inertia switch (some cars), tank connector.
- Pressure low → filter restriction, weak pump, low voltage at pump, regulator issue, leak-back.
- Pressure OK → proceed to injection + spark.
Use: Fuel pressure testing basics →
Injection command
If you have pressure but no start, confirm injectors are being commanded (noid light / scan tool pulse width). No command can be immobiliser, missing cam signal, ECU power/ground, or safety cut.
Spark / ignition
- No spark → check ignition power feeds, crank/cam sync, coil driver, ECU grounds, relevant fuses.
- Spark present → move to air + compression + timing.
Air / throttle / massive restriction
Ensure intake isn’t blocked, throttle plate isn’t stuck closed, and there isn’t a catastrophic restriction (collapsed intake pipe, severe exhaust blockage). If you suspect exhaust restriction, see: P0420 principles.
Compression + mechanical timing plausibility
- Very low/uneven compression → mechanical issue (timing belt/chain slip, washed cylinders, valve damage).
- Compression OK but still no start → look harder at timing correlation and sensor plausibility.
Use: P0016/P0017 timing correlation →
When it finally starts: capture the story
Save freeze-frame, note whether it only starts with throttle, only cold/hot, after sitting, etc. That pattern usually narrows the root cause.

Related flows

Print / save checklist

Tick these off as you work. If you need to hand this to a mechanic, print it as a short job card.

Freeze-frame captured / conditions noted
Battery voltage checked (resting + cranking)
Basic visual checks (hoses, connectors, grounds, fuses)
One test at a time (don’t change multiple variables)
Confirm fix by reproducing the original condition

Print / Save as PDF Back to symptom selector

What to do next

Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.

Find another symptom flow Jump to the symptom selector to locate the closest decision tree. Workshop Guides Deep-dive how-to tests: voltage drop, wiring diagrams, smoke testing, fuel pressure and more. Diagnostic Codes Look up DTC meanings, common causes, and related checks. AI Tools Use AI assistance to summarise symptoms, plan tests, or sanity-check a diagnosis.