An engine that fires and then stops narrows the diagnosis differently from one that never fires.
Why this matters
Some immobilizer systems allow a brief start before fuel or spark is disabled. A failing pump may also build enough prime pressure for a few seconds then lose it.
An electronic throttle fault can prevent stable idle even though combustion begins.
First checks
• Observe security indicator.
• Watch fuel pressure after initial start.
• Check throttle/idle air control strategy.
• Look for large intake leaks or disconnected ducts.
• Check whether key/start authorization remains valid after initial firing.
A practical test path
Begin with the easiest observation that can divide the problem into two paths. In a starts and dies immediately complaint, do not change several things at once. Record the initial condition, perform one test, and then decide what the result proves. If the test shows normal power, move downstream. If it shows a loss, stay with that part of the circuit until the source of the loss is found. Whenever possible, test the circuit while it is carrying its normal load rather than relying only on resistance or continuity checks with everything switched off.
For a crank-no-start, use scan data and direct tests to determine which basic requirement is missing. Normal starter speed does not prove the engine computer has a valid position signal, correct fuel pressure, ignition output, authorization, or compression.
Common mistakes to avoid • Holding the throttle wide open as a universal fix.
Stop and get help when… • If the engine races unexpectedly or throttle control is abnormal, stop testing and investigate safely.
What to record
Write down whether the symptom was present during the test, the battery voltage or scan values you observed, and exactly what changed. For starts and dies immediately, include temperature, key used, whether the engine cranked, and any warning or security indicators that were different from normal. Good notes prevent repeated testing and make professional diagnosis faster if you need it.