The crankshaft position sensor (CKP) is one of the most critical engine sensors.
If it fails or gives incorrect signals, your car may crank but not start, stall suddenly, or run rough, because the ECU can’t properly time fuel injection and spark.
Let’s go through this in complete detail, including how the sensor works, common symptoms of failure, diagnostic methods, and how to confirm the fault.
1. What the Crankshaft Position Sensor Does
The crankshaft position sensor (CKP) monitors the exact position and rotational speed (RPM) of the crankshaft.
It sends a signal to the engine control unit (ECU), which uses that data to:
- Determine when to fire the spark plugs
- Time fuel injection precisely
- Detect engine speed (RPM)
- Coordinate with the camshaft position sensor for proper ignition timing
Without a correct CKP signal, the ECU can’t synchronize ignition or injection → engine won’t run.
2. Types of Crankshaft Sensors
| Type | Description | Signal Type |
|---|---|---|
| Magnetic (inductive) | Has coil and magnet; generates AC voltage as crank teeth pass by | AC waveform |
| Hall effect | Has a transistor that switches on/off based on magnetic field | Digital square wave |
| Optical (rare) | Uses light interruption; older systems | Digital |
3. Common Symptoms of a Bad Crankshaft Position Sensor
Let’s go through the key warning signs, in detail.
1. Engine Cranks but Won’t Start
Description:
- Starter turns the engine normally, but it never fires or starts.
- There is no spark and no injector pulse.
Why:
- ECU doesn’t receive the crank signal → doesn’t know when to fire spark plugs or inject fuel.
Confirm:
- OBD-II scanner shows 0 RPM while cranking.
- Code P0335 (Crankshaft Position Sensor “A” Circuit) often appears.
2. Engine Stalls While Driving
Description:
- Car suddenly shuts off while running, especially when hot.
- After cooling for 10–15 minutes, it restarts.
Why:
- CKP sensor fails when hot (internal coil opens with heat).
- Once it cools, the circuit closes again → works temporarily.
Confirm:
- No spark or injector pulse during stall condition.
- Crank signal returns after cooling.
3. Intermittent Starting Problems
Description:
- Sometimes starts normally, other times cranks with no fire.
- Seems random or worse when hot.
Why:
- Sensor intermittently losing signal (loose connector, damaged wire, or internal short).
4. Check Engine Light (CEL)
Description:
- CEL comes on; scanning shows CKP-related fault codes.
Common Codes:
| Code | Meaning |
|---|---|
| P0335 | CKP Sensor “A” Circuit Malfunction |
| P0336 | CKP Range/Performance |
| P0337 | CKP Low Input |
| P0338 | CKP High Input |
| P0339 | CKP Intermittent |
5. Engine Misfires or Rough Idle
Description:
- Engine stumbles or runs unevenly, especially at low speeds.
Why:
- Weak or inconsistent CKP signal causes poor ignition timing.
6. Tachometer Drops to Zero While Engine Running
Description:
- RPM gauge suddenly drops to 0, even though engine is still spinning.
Why:
- ECU momentarily loses crank signal — tachometer uses same input.
7. Poor Acceleration or Reduced Power
Description:
- ECU can’t accurately time ignition → sluggish response or hesitation.
8. No Spark / No Fuel Injector Pulse
Description:
- When cranking, spark plugs have no spark, and injectors don’t click.
Why:
- ECU disables both systems without crankshaft position reference.
4. How to Diagnose a Bad Crankshaft Position Sensor
You can test the CKP sensor using basic tools like a multimeter or scan tool, and sometimes an oscilloscope for waveform analysis.
Step 1: Check for Trouble Codes
Tool: OBD-II scanner
- Plug into diagnostic port.
- Look for codes P0335–P0339.
- If found → CKP signal problem confirmed.
If no codes but car won’t start → proceed to live data.
Step 2: Check RPM While Cranking
Tool: OBD-II scanner (Live Data)
- Observe “Engine RPM” while cranking.
- If RPM stays at 0 → ECU not receiving signal → bad sensor or wiring.
- If RPM fluctuates normally (100–300 RPM) → CKP working.
Step 3: Check Power, Ground, and Signal Wires (for Hall sensors)
Tool: Multimeter
- Identify CKP connector (usually 3 wires):
- +5V reference
- Ground
- Signal output
- Key ON:
- +5V between reference and ground.
- 0V–5V switching on signal wire when cranking (for Hall sensor).
- If no reference voltage → wiring or ECU fault.
- If no signal output → sensor faulty.
Step 4: Test Resistance (for Magnetic/Inductive Sensors)
Tool: Multimeter (Ohms setting)
- Disconnect CKP connector.
- Measure resistance across the two terminals:
- Typical range: 200–1,000 Ω (check manufacturer spec).
- Infinite (open circuit) or 0 Ω (short) = bad sensor.
- Optional: Set multimeter to AC volts.
- Crank the engine → should generate 0.2–1.5V AC.
- No voltage = failed sensor.
Step 5: Inspect Wiring and Connector
- Look for:
- Frayed wires
- Oil contamination
- Corroded or loose pins
- Sensor knocked out of alignment
A poor connection can mimic sensor failure.
Step 6: Oscilloscope Test (Advanced)
- Observe waveform of CKP signal while cranking.
- Healthy sensor: steady, repeating wave (sine wave or square wave).
- Missing peaks or noise → faulty sensor or damaged reluctor wheel.
5. How to Fix a Bad Crankshaft Position Sensor
If diagnosis confirms the sensor is bad:
Tools You’ll Need:
- Socket set & ratchet
- Jack and stands (if under engine)
- Screwdriver
- New CKP sensor
- Torque wrench
- Dielectric grease
Steps:
- Disconnect battery (negative terminal).
- Locate the sensor:
- Often near crank pulley, flywheel, or transmission bell housing.
- Disconnect the electrical connector.
- Remove the retaining bolt(s) and pull out the sensor gently.
- Some may be stuck with grime; avoid prying too hard.
- Clean mounting hole and check the reluctor ring/teeth for damage or debris.
- Install the new sensor:
- Ensure proper alignment and air gap (usually 0.5–1.0 mm).
- Torque bolt to manufacturer spec.
- Reconnect connector and battery.
- Clear codes with scanner.
- Test start — engine should crank and fire immediately.
6. Common Mistakes to Avoid
| Mistake | Consequence |
|---|---|
| Installing sensor without cleaning metal debris | False signal readings |
| Damaging sensor wire harness | Intermittent failure |
| Forgetting to reset ECU codes | CEL remains on |
| Not verifying reluctor wheel condition | Repeated failure |
| Mixing up crank and cam sensor connectors (on some engines) | No start condition |
7. Summary — Signs of a Bad Crankshaft Sensor
| Symptom | Description | Confirmation |
|---|---|---|
| Cranks but no start | No spark / no injector pulse | 0 RPM on scan tool |
| Engine stalls when hot | Sensor fails under heat | Restarts when cool |
| CEL with P0335–P0339 | ECU detects bad signal | Scan for DTCs |
| Tachometer drops to zero | Signal loss | Observe dash RPM |
| Misfires / poor idle | Inconsistent signal | Scope test |
| No voltage / open resistance | Internal short or break | Multimeter test |
8. Pro Tip
Many “bad crank sensor” symptoms can also be caused by:
- Damaged wiring harness near exhaust or bell housing
- Bad camshaft sensor (especially in systems that use both)
- Poor ECU ground
- Timing chain or reluctor wheel damage
Always check wiring and mechanical timing before replacing the sensor.
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