The camshaft position sensor (CMP) is crucial for timing ignition and fuel injection. When it starts failing, it can seriously affect engine performance. Here are the 7 most common symptoms of a bad camshaft position sensor, explained in detail:
In this article:
7 symptoms of a bad camshaft position sensor?
1️⃣ Check Engine Light (CEL) Illuminates
- The ECU monitors the CMP sensor continuously.
- If it detects irregular or no signal, it triggers the Check Engine Light.
- Typical codes:
- P0340 – CMP Sensor Circuit Malfunction
- P0341 – CMP Sensor Range/Performance
- CEL may be steady or flashing depending on severity.
2️⃣ Engine Misfires or Rough Idle
- The CMP sensor tells the ECU camshaft position relative to crankshaft, crucial for spark timing.
- A failing sensor can cause:
- Cylinder misfires
- Rough idling
- Engine vibrations
- Misfires are often intermittent, especially under load.
3️⃣ Engine Stalling or Hard Starting
- Weak or intermittent CMP signal can cause the engine to stall suddenly or crank but not start.
- Often happens when the engine is warm, because heat affects sensor electronics.
4️⃣ Poor Acceleration or Reduced Power
- The ECU may retard ignition timing or limit fuel delivery to protect the engine when CMP data is erratic.
- Symptoms include:
- Hesitation when pressing the accelerator
- Sluggish throttle response
- Surging or jerky acceleration
5️⃣ Engine Won’t Start
- In severe CMP failure, the ECU cannot determine when to fire spark or inject fuel.
- The result: Engine cranks but refuses to start.
- Often confused with crankshaft sensor or fuel system issues.
6️⃣ Poor Fuel Economy
- Erratic CMP signals cause the ECU to miscalculate fuel injection timing.
- Leads to unburned fuel and reduced MPG.
- You may also notice black smoke from the exhaust due to incomplete combustion.
7️⃣ Transmission Shifting Issues (On Some Cars)
- Modern vehicles with ECU-controlled transmissions use CMP data for shift timing.
- Symptoms:
- Hard or delayed gear shifts
- Transmission stuck in limp mode
- Particularly common in vehicles with variable valve timing (VVT) systems.
✅ Summary Table of Symptoms
| # | Symptom | Why It Happens |
|---|---|---|
| 1 | Check Engine Light | ECU detects faulty or no CMP signal |
| 2 | Misfire / Rough Idle | Incorrect ignition timing per cylinder |
| 3 | Engine Stalling / Hard Start | ECU receives erratic signal |
| 4 | Poor Acceleration | Ignition and fuel timing are compromised |
| 5 | Engine Won’t Start | ECU cannot time spark or fuel injection |
| 6 | Poor Fuel Economy | Timing errors → incomplete combustion |
| 7 | Transmission Shifting Issues | CMP data affects VVT and ECU shift logic |
💡 Pro Tip:
A failing camshaft sensor often mimics crankshaft sensor issues, ignition problems, or fuel delivery faults. Always check both sensors and scan for codes before replacing parts.
Other courses:
