When the engine fires up but immediately dies, or runs for 2-5 seconds and quits, the engine got enough fuel and spark to start - but lost one of them right after. This pattern points to a failed crankshaft position sensor, a stuck idle air control valve, an anti-theft / immobilizer rejection, a leaking fuel pressure regulator, or a vacuum leak that prevents idle. Each has a specific signature.
Pay attention to how long it runs. Dies instantly (under 1 second) usually means anti-theft. Runs 2-5 seconds then dies usually means crank sensor or fuel pressure. Runs and surges before dying usually means idle air control or vacuum leak.
Each cause is rated by likelihood, repair cost range, DIY difficulty, and severity. Start with the highest-probability cause and work down.
The PCM uses initial cam signal to start, then needs crank signal to keep running. If the crank sensor fails or its wiring is intermittent, the engine fires briefly and then dies. Often heat-related.
If the immobilizer does not recognize the key, the PCM allows a 2-second start and then kills the fuel pump. Look for a flashing security/lock light on the dash. Try the spare key.
On older engines without electronic throttle, the IAC valve controls idle air. When stuck closed, idle drops to zero as soon as the starter releases. Clean with throttle body cleaner or replace.
A regulator that bleeds off pressure between starts has just enough for initial fire, but pressure drops below operating spec and the engine dies. Look for fuel in the vacuum line at the regulator.
Cracked intake hose, broken vacuum line, blown PCV. So much unmetered air enters that the PCM cannot maintain idle. Listen for hissing, spray carb cleaner around suspect joints.
Dirty or failed MAF sends bad data to the PCM. The PCM enriches fuel for start but cannot calculate correct idle. Clean MAF with MAF cleaner only.
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If your scan tool is showing one of these codes alongside this symptom, that is your starting point. Click any code for the full diagnosis.
The engine had enough fuel and spark to fire, but lost one immediately. Top causes are a bad crankshaft sensor (loses position signal), anti-theft immobilizer (cuts fuel after 2 seconds), stuck idle air control valve (idle drops to zero), or a failing fuel pressure regulator.
Watch the dashboard. If a key/lock/security icon flashes when you turn the key, the immobilizer is rejecting your key. Try the spare key. If the spare works, the chip in your primary key has failed - reprogram or replace.
Yes. Carbon buildup behind the throttle plate restricts airflow so much that idle cannot sustain. Clean the throttle body with throttle body cleaner and a rag. $5 and 20 minutes. Often solves the problem completely on older cars.
The regulator maintains fuel rail pressure. When the diaphragm leaks, fuel bleeds back to the tank between drives. Initial start works because the pump primes it, but pressure drops below spec immediately and the engine starves. Check for fuel in the vacuum line at the regulator.
Possible but less common than the others. A pump that primes but loses pressure under run conditions would do this. A fuel pressure gauge on the schrader valve confirms - should hold 30-60 PSI (varies by vehicle) at idle.
Crank sensor: $100-250. IAC valve clean/replace: $20-200. Throttle body cleaning: $5-50 DIY. Fuel pressure regulator: $50-300. Vacuum leak repair: $20-300. MAF cleaning: free, replacement $80-300.