Car Won’t Start (Battery OK): Causes, Likely Codes & What to Do
TL;DR
Car won’t start but battery OK = the engine cranks but won’t fire (or won’t crank with full electrics). Severity: high. Likely codes: P0335 (crank sensor), P0340 (cam sensor), U0100 (lost comm with ECM). Top causes: crank sensor, fuel pump, immobilizer, ECM communication. Scan to confirm.
What "car won’t start (battery ok)" means
A good battery rules out the most common no-start cause, so attention shifts to what the engine needs to fire: fuel pressure, a spark, and a crank/cam position signal so the computer knows when to inject and ignite. If the engine cranks but never catches, one of those is missing. If it won’t crank at all despite good electrics, suspect the starter, a security lockout, or a communication fault.
Quick diagnosis: IF → THEN
| If… | Then… |
|---|---|
| the engine cranks normally but never fires | suspect no fuel or no spark — check the fuel pump/pressure and the crank sensor (P0335) |
| it won’t crank but lights and dash are strong | suspect the starter, a security/immobilizer lockout, or a neutral-safety/clutch switch |
| the dash shows many warning lights or a security message | suspect an immobilizer or communication fault (U0100) |
| it almost starts then dies | suspect fuel pressure or a cam/crank correlation issue (P0340) |
Most likely fault codes
| Code | Likelihood | Notes |
|---|---|---|
| P0335 — Crankshaft position sensor | 35% | No crank signal = no start |
| P0340 — Camshaft position sensor | 25% | |
| U0100 — Lost communication with ECM | 25% | |
| P2293 — Fuel pressure regulator | 15% |
Common causes
- Failing crankshaft or camshaft position sensor (no start signal)
- Dead or weak fuel pump / no fuel pressure
- Security / immobilizer fault (key not recognized)
- Lost communication with the engine computer (U0100)
- Faulty starter or starting-circuit safety switch
What to do
- Confirm the battery really is good (strong cranking, bright lights).
- Scan for codes — a crank/cam sensor or communication fault often shows up.
- Listen for the fuel pump priming when you turn the key to ON.
- Check for a security/immobilizer light or message.
- If it cranks but won’t fire, test for spark and fuel pressure.
When is it urgent?
A no-start usually leaves you stranded rather than in danger, but if it happens in an unsafe location, prioritize getting to safety. Otherwise, scan and work through fuel, spark and sensor checks.
Frequently asked questions
My battery is fine but the car won’t start — what is it?
With a good battery, the usual culprits are a failing crankshaft position sensor, a dead fuel pump, an immobilizer/security fault, or lost communication with the engine computer. Scanning for codes is the fastest way to narrow it down.
Can a crankshaft sensor stop a car from starting?
Yes. Without a crank signal the computer will not fire injectors or spark, so the engine cranks but never starts — often with code P0335.
Why does my car crank but not start?
Cranking means the battery and starter are working, so the missing ingredient is fuel, spark, or a position signal. Check fuel pressure, the crank/cam sensors, and for a security lockout.