Modern vehicles have multiple sensors that the engine control unit (ECU) depends on to allow or inhibit starting.
If any of these sensors fail or send incorrect signals, the ECU may block ignition, fuel injection, or even the starter relay, making the car crank but not start — or not crank at all.
Let’s go through this in full detail, including which sensors affect starting, what symptoms they cause, and how to diagnose them.
1. Overview: What “Stops a Car from Starting”
When you turn the key (or press the start button), your car’s computer expects certain sensor signals before it allows the engine to:
- Crank (starter motor engage)
- Inject fuel
- Fire spark plugs
If any critical signal is missing or invalid, the ECU stops the process for safety or engine protection.
Also watch: Engine cranks but wont start common reasons.
2. Main Sensors That Can Prevent a Car from Starting
Let’s divide them by system.
- Crankshaft Position Sensor (CKP)
- Camshaft Position Sensor (CMP)
- Engine Coolant Temperature Sensor (ECT)
- Mass Air Flow Sensor (MAF) / Manifold Absolute Pressure Sensor (MAP)
- Throttle Position Sensor (TPS)
- Immobilizer / Key Transponder Sensor
- Neutral Safety Switch (Automatic) / Clutch Pedal Switch (Manual)
- Accelerator Pedal Position Sensor (Drive-by-Wire)
- Fuel Pressure Sensor
- Oxygen Sensors (O₂ / Lambda Sensors)
A. Crankshaft Position Sensor (CKP)
Function:
- Detects the exact position and speed (RPM) of the crankshaft.
- ECU uses this data to time fuel injection and spark.
If It Fails:
- The engine will crank, but won’t start — because ECU doesn’t know when to fire spark or inject fuel.
Symptoms:
- Cranks but no start
- No RPM signal on scan tool while cranking
- No spark at plugs
- Check Engine Light may show P0335 (Crankshaft Position Sensor “A” Circuit)
Diagnostic Tip:
- Measure AC voltage from CKP while cranking (should produce small signal).
- If no signal, or signal erratic → replace sensor.
B. Camshaft Position Sensor (CMP)
Function:
- Works with crank sensor to determine which cylinder is firing.
- Needed for sequential fuel injection and ignition timing.
If It Fails:
- In many engines, car may crank longer or not start at all.
- ECU may switch to “batch injection” mode (fires injectors in groups).
Symptoms:
- Cranks but no start
- Backfiring or rough starting
- Check Engine Code: P0340 (Camshaft Position Sensor Circuit)
C. Engine Coolant Temperature Sensor (ECT)
Function:
- Measures engine temperature for proper fuel mixture during startup.
If It Fails:
- ECU may think the engine is extremely hot or cold → gives wrong fuel mixture.
- Too rich (flooded) or too lean (no start).
Symptoms:
- Cranks but no start (especially cold)
- Hard starting
- Black smoke or fuel smell from exhaust
- Code: P0115–P0119
D. Mass Air Flow Sensor (MAF) / Manifold Absolute Pressure Sensor (MAP)
Function:
- MAF: Measures air entering engine.
- MAP: Measures intake manifold pressure (used in speed-density systems).
If It Fails:
- ECU can’t calculate proper fuel amount → mixture incorrect → may not start.
- Usually causes crank but no start, especially if signal completely dead.
Symptoms:
- Cranks but doesn’t fire
- Starts and stalls immediately
- Unstable idle or surging
- Codes: P0100–P0104 (MAF) or P0105–P0109 (MAP)
E. Throttle Position Sensor (TPS)
Function:
- Reports throttle opening angle to ECU.
- Used for fuel enrichment during cranking and idle control.
If It Fails:
- Some ECUs disable fuel injection during crank if throttle signal stuck open (anti-flood protection).
Symptoms:
- Cranks but won’t start unless pressing pedal
- Erratic idle
- Code: P0120–P0124
F. Immobilizer / Key Transponder Sensor
Function:
- Security sensor reads the RFID chip in your key and communicates with ECU.
- Prevents unauthorized engine start.
If It Fails:
- Engine may crank but immediately die, or not crank at all.
- Dash may show security light flashing.
Symptoms:
- “Key” or “Padlock” light flashing on dashboard
- Starts then shuts off in 1–2 seconds
- No Check Engine Light activity when key on
G. Neutral Safety Switch (Automatic) / Clutch Pedal Switch (Manual)
Function:
- Ensures car only starts in Park/Neutral (automatics) or when clutch pedal pressed (manuals).
If It Fails:
- Starter won’t engage at all (no crank).
Symptoms:
- No crank when turning key
- Car starts only in certain gear (wiggling shifter helps)
- Code: P0705 (Transmission Range Sensor)
H. Accelerator Pedal Position Sensor (Drive-by-Wire)
Function:
- In modern cars with electronic throttle, ECU needs this sensor to know pedal input.
If It Fails:
- Some ECUs prevent start for safety (no throttle signal = “limp” mode or no start).
Symptoms:
- No start, or starts and stalls
- Throttle doesn’t respond
- Code: P2122–P2128
I. Fuel Pressure Sensor
Function:
- Monitors fuel rail pressure; ECU ensures sufficient fuel pressure before allowing injection.
If It Fails:
- ECU may disable injectors or fuel pump relay → no start.
Symptoms:
- Long crank or no start
- No fuel pressure (0 psi)
- Code: P0190–P0193
J. Knock Sensor (Indirectly)
Function:
- Detects engine knock or detonation for timing adjustment.
If It Fails:
- Normally won’t stop starting, but some ECUs inhibit start if readings are out of range (rare).
K. Oxygen Sensors (O₂ / Lambda Sensors)
Function:
- Used for air–fuel ratio correction after startup (not for starting).
If It Fails:
- Won’t usually prevent starting — but can cause stalling after a few seconds.
3. Less Obvious Sensors and Devices That Can Stop Starting
| Component | Function | Failure Effect |
|---|---|---|
| Brake pedal switch | Required for push-button start | No crank if brake not sensed |
| Inertia/fuel cutoff switch (crash sensor) | Cuts fuel in accident | No fuel → no start |
| ECU main relay | Powers sensors/injectors | No power → total no-start |
| MAP/MAF wiring short | Pulls down 5V reference line | Disables multiple sensors → no start |
| Oil pressure sensor (rare) | Some ECUs cut fuel at zero pressure | No start (safety shutdown) |
4. Quick Diagnostic Steps
| Step | Check | What It Tells You |
|---|---|---|
| 1 | Does engine crank? | If no → check starter circuit (neutral/clutch sensor, ignition switch) |
| 2 | Does fuel pump prime (2-sec hum)? | If not → fuel system or immobilizer issue |
| 3 | Is there spark? | No spark → suspect crank/cam sensors |
| 4 | Scan for trouble codes (OBD-II) | Missing signals from sensors show up as P0xxx codes |
| 5 | Check security light | Flashing = immobilizer fault |
| 6 | Check live data (RPM, temp, throttle) | Missing readings = sensor fault |
5. Summary: Sensors That Can Stop a Car from Starting
| Sensor | Engine Cranks? | Effect | Common Code |
|---|---|---|---|
| Crankshaft Position (CKP) | ✅ | No spark, no fuel | P0335 |
| Camshaft Position (CMP) | ✅ | Misfire or no start | P0340 |
| Coolant Temp (ECT) | ✅ | Wrong fuel mix | P0115 |
| MAF / MAP | ✅ | Wrong air–fuel ratio | P0100 / P0105 |
| Throttle Position (TPS) | ✅ | Flooded or no injection | P0120 |
| Fuel Pressure | ✅ | No fuel to injectors | P0190 |
| Immobilizer / Key Sensor | 🚫 / ✅ | No crank or start, engine dies | — |
| Neutral Safety / Clutch Switch | 🚫 | No crank | P0705 |
| Brake Pedal Sensor | 🚫 | No crank (push start) | P0571 |
| ECU Main Relay / Crash Sensor | 🚫 | No ECU power or fuel cut | — |
6. How Mechanics Usually Find the Fault
- Connect a scan tool (OBD-II) → check for missing sensor signals.
- Observe live data while cranking:
- RPM = 0 → Crank sensor dead.
- Temp reading unrealistic (e.g. –40°C) → ECT sensor bad.
- Throttle stuck at 100% → TPS fault.
- Check power supply and ground to ECU.
- Check immobilizer/security light.
- Test with starter relay bypass (to rule out neutral/clutch sensors).
7. Summary: The Most Common “No Start” Sensor Failures
| Rank | Sensor | Typical Symptom |
|---|---|---|
| 1 | Crankshaft Position Sensor | Cranks but never fires |
| 2 | Camshaft Position Sensor | Long crank or no start |
| 3 | Immobilizer / Key Sensor | Starts then dies, or no crank |
| 4 | Neutral Safety / Clutch Switch | No crank at all |
| 5 | Coolant Temp Sensor | Hard cold start or no start |
| 6 | MAF/MAP Sensor | Crank but no fire or instant stall |
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