The crankshaft position sensor (CKP) is one of the most critical engine sensors, and when it starts failing, the symptoms can be subtle at first and escalate quickly. Understanding these signs is key because a bad CKP can cause your car to stall, misfire, or not start at all.
Here’s a detailed breakdown:
What the Crankshaft Sensor Does
- Monitors the rotational position and speed of the crankshaft.
- Sends this information to the Engine Control Unit (ECU).
- ECU uses it to:
- Time ignition spark
- Control fuel injection
- Synchronize camshaft timing (if variable timing is used)
Without a reliable CKP signal, the ECU cannot run the engine properly.
Common Symptoms of a Failing Crankshaft Sensor
1️⃣ Engine Cranks but Won’t Start
- Symptom: You turn the key, starter motor spins, but the engine does not fire.
- Why: ECU doesn’t detect crankshaft rotation → no spark, no injector pulse.
- Check: OBD-II scanner may show code P0335 (Crankshaft Position Sensor “A” Circuit Malfunction).
2️⃣ Intermittent Stalling
- Symptom: Engine suddenly dies while driving and may restart after a few minutes.
- Why: The sensor fails intermittently, often worsening with heat.
- Tip: If the engine starts after cooling down, CKP is a likely culprit.
3️⃣ Engine Misfires or Rough Idle
- Symptom: Engine runs unevenly, surges, or vibrates at idle.
- Why: Erratic CKP signals confuse the ECU’s ignition timing.
4️⃣ Check Engine Light (CEL)
- Symptom: CEL illuminates; codes like:
- P0335 – CKP Sensor “A” Circuit Malfunction
- P0336 – CKP Range/Performance
- P0337 – CKP Low Input
- P0338 – CKP High Input
- P0339 – CKP Intermittent
5️⃣ Tachometer Drops to Zero
- Symptom: RPM gauge drops suddenly while engine is running.
- Why: ECU temporarily loses CKP signal; tachometer uses the same input.
6️⃣ Poor Acceleration / Reduced Power
- Symptom: Hesitation or sluggish engine response.
- Why: Timing errors due to missing or inconsistent CKP signal.
7️⃣ No Spark / Fuel Injector Pulse
- Symptom: Spark plugs don’t fire; injectors don’t click.
- Why: ECU disables ignition and fuel when it can’t detect crankshaft rotation.
8️⃣ Engine Starts Only Intermittently
- Symptom: Sometimes engine starts normally; other times it won’t.
- Why: Sensor signal is weak or inconsistent.
How to Confirm a Bad Crankshaft Sensor
- Listen for Codes: Scan OBD-II codes related to CKP.
- Check Engine RPM While Cranking:
- 0 RPM → no signal → possible CKP failure
- Test Sensor with Multimeter:
- For inductive type: check resistance (typically 200–1000 Ω).
- For Hall-effect: check 5V reference, ground, and signal output.
- Visual Inspection:
- Look for damaged wires, corrosion, oil contamination, or misalignment.
- Oscilloscope Test (Advanced):
- Observe waveform during cranking; irregular or missing peaks = bad sensor.
Summary — Key CKP Failure Symptoms
| Symptom | What It Indicates |
|---|---|
| Cranks but won’t start | No CKP signal → no spark/fuel |
| Intermittent stalling | Sensor failing, worse when hot |
| Rough idle / misfire | Erratic timing due to poor signal |
| CEL codes P0335–P0339 | ECU detects CKP circuit issues |
| Tachometer drops to 0 | Momentary signal loss |
| Poor acceleration | Timing irregularities |
| Engine starts intermittently | Weak or inconsistent signal |
💡 Pro Tip: Many issues that seem like fuel or ignition problems — like stalling, rough idle, or hard starting — can actually be caused by a failing crankshaft sensor. Always check CKP signal before replacing other components.
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