Diagnostic Case Study

Crank-No-Start with Dead Headlights & Signals

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A 2020 Jaguar XF (Diesel) cranked but never caught, whilst its headlights and indicators were totally dead. It was tempting to blame one common electrical cause.

The Challenge

A dramatic symptom like dead exterior lights can easily look related to a no-start condition. Many mechanics would assume a single massive electrical failure, like a fried Body Control Module (BCM) or a severed main earth strap.

The workshop needed a methodical way to separate these symptoms and trace them to their root causes without getting distracted by the coincidence.

Structuring the Diagnostic

By adopting a structured and documented approach, the workshop could have used the EVMetricsOBD Workshop Templates to guide the diagnostic process. These templates lead the technician through a strict hierarchy of tests, ruling out causes logically and preventing expensive misdiagnoses.

Crank-No-Start Diagnostic

Engine cranks but won't fire — work fuel / air / compression / ignition / sensors / security / electrical
  • Confirm cranking speed & the basics
    Normal cranking speed but no start? Slow/laboured cranking instead points at battery/starter/earths. Check there's fuel in the tank, note any dash/immobiliser warnings, and what changed (cold vs hot, after a fill-up, after recent work).
  • Scan all modules for codes & freeze frame
    Read DTCs across modules plus any freeze-frame. Stored crank/cam, fuel-pressure, glow, or immobiliser codes steer the whole search before you start testing.
  • Does the ECU see the engine turning?
    Watch live RPM while cranking. No RPM = the crank/cam position signal is missing and the ECU won't fire fuel or spark. RPM present = move on to fuel / spark / compression.
  • Immobiliser / start authorisation
    Confirm the key is recognised and security isn't blocking start: immobiliser lamp, key/fob battery, smart-key range, and BCM/gateway start authorisation.
  • Fuel supply & pressure
    Hear the pump prime on key-on; measure fuel pressure during crank — low-side, and the high-pressure rail on DI/diesel. Check the filter and that the tank is actually feeding.
  • Fuel quality / contamination
    Stale petrol, water or diesel-bug, or a misfuel (petrol in a diesel) will crank-no-start. Suspect especially right after a fill-up.
  • Injectors firing
    Confirm injector pulse/clicking while cranking. On diesel, a leaking/stuck injector can bleed off rail pressure so the engine never builds enough to fire.
  • Spark (petrol)
    Verify spark at a plug while cranking; check coils, plugs and the ignition feed. No spark with an RPM signal points at ignition, not the sensor. Mark N/A on diesels.
  • Glow / cold-start aids (diesel)
    Check the glow-plug relay/module and plug operation (and any intake heater). A hard cold-start no-start that improves when warm often lives here. Mark N/A on petrol engines.
  • Air intake — rule out a gross restriction
    Collapsed intake hose, blocked filter, throttle stuck shut, or a MAF/airflow fault skewing the mixture far enough to stop it firing.
  • Compression & timing (mechanical)
    A snapped or jumped timing belt/chain cranks fast and won't start. Compression / leak-down confirms cylinder sealing and cam-to-crank correlation.
  • Crank & cam position sensors
    Test the sensors directly (resistance/scope) — especially for a hot no-start that recovers once cold (heat-soak sensor dropout).
  • Electrical supply (one path of several)
    If cranking is weak OR other electrics misbehave (dead lights, modules reporting garbage), check 12V supply, main grounds/earths, distribution (fuses/links/BJB) and BCM supply. Expand with the Fuse & Relay Check template.
  • Decision / escalate
    Narrow to fuel vs air vs compression vs ignition/glow vs position sensor vs security vs electrical, recorded against the live-data and test results.
The Outcome The Crank-No-Start template correctly structured the physical observations: the fast, free cranking indicated no compression, and a static camshaft proved the chain wasn't transmitting drive. This confidently separated a mechanical timing chain failure from a completely independent BCM/power distribution fault.