📋 Quick Facts
Time
15 minutes
Difficulty
Moderate
Tools
3 tools
Cost to DIY
$25-50
The CMP sensor tells the ECU where the camshaft is - which valves are open. Combined with the CKP, the ECU knows exactly which cylinder needs spark and fuel next. A bad CMP usually causes long crank, missed firing, and sometimes a no-start when the engine is hot.
🛠 What You'll Need
- Digital multimeter (AC + DC + ohms) (shop a digital multimeter on Amazon)
- OBD2 scanner (shop a Bluetooth OBD2 scanner on Amazon)
- Back-probe pins (shop back-probe pin set on Amazon)
- Service manual / wiring diagram
Product links above are Amazon affiliate links. AmpAuto earns a small commission at no extra cost to you.
⚠ When NOT to DIY thisAvoid working near the timing belt/chain. If the sensor is on the back of the cylinder head, removing/installing it can be tight - do not drop a bolt into the engine bay where you cannot retrieve it.
🎯 Expected Readings (Pass/Fail Reference)
| Inductive (2-wire) CMP resistance | 200 - 1,500 ohms typical |
| Hall (3-wire) supply voltage | 5 V (or 12 V on some) |
| Hall signal during crank | Pulses 0 - 5 V (or 0 - 12 V) |
| Inductive AC during crank | 0.2 - 2 V AC |
| Sensor air gap | 0.020" - 0.060" if adjustable |
| CMP signal on scan tool | Visible "cam sensor present/active" during crank |
Numbers are typical. Always cross-check against your factory service manual for the exact spec.
📝 Step-by-Step Test Procedure
- Pull CMP codesP0340 (CMP circuit), P0341 (range/performance), P0342 (low), P0343 (high), P0344 (intermittent), P0345-P0349 (Bank 2 versions). Cross-check with P0335-series in case crank sensor is also involved.
- Visual inspectionLocate the sensor (top of valve cover, side of cylinder head, or in timing cover - depends on engine). Look for oil on the connector, cracked housing, or a damaged wire.
- Identify 2-wire vs 3-wire2-wire = inductive (passive). 3-wire = Hall-effect (active). Test method differs.
- Inductive resistance testUnplug, set multimeter to 2000 ohms. Probe both pins. 200-1,500 ohms = good. Infinity or near 0 = bad.
- Hall sensor supply checkKey on, engine off. Back-probe between supply pin and ground at the sensor connector. Should read 5V or 12V steady.
- Hall sensor signal testPlugged in. Back-probe the signal wire. Multimeter on DC volts. Crank or run engine - voltage should pulse between 0 V and 5 V (or 12 V). Flat reading = bad sensor.
- Heat soak test (intermittent failure)If the car only stalls when hot: with a hair dryer or heat gun, warm the sensor body to 150-180°F while watching the multimeter. If signal drops or disappears, the sensor fails when hot - replace.
- Replace and clear codesSome cars require a relearn (scan tool needed). Match OEM brand - aftermarket CMPs have a higher failure rate than CKPs. shop camshaft position sensors on Amazon.
✅ Pass / Fail Criteria
✅ PASS
Correct resistance or 5V/12V supply, pulsed signal during crank, no oil contamination, no heat-soak failure
❌ FAIL
Out-of-range resistance, no supply voltage at sensor, flat signal during crank, or signal disappears when warmed
🔧 If It Fails - What To Do Next
Replace the CMP sensor. Some engines require resealing the sensor housing with a new O-ring. See What is a cam position sensor? and Why won't my car start?
🔗 Related Guides
❓ Frequently Asked Questions
What does a bad cam sensor cause?
Long crank, no-start when hot, occasional misfires, and a P0340-series code. Many cars will still start in limp mode using just the crank sensor.
Can I drive with a bad CMP sensor?
Yes, but the engine runs in batch-fire mode - reduced MPG, possible rough running. Replace soon.
How is a CMP different from a CKP?
CMP reads the camshaft; CKP reads the crankshaft. CMP timing is once per 2 engine revolutions; CKP is once per revolution.
Why does the CMP fail when hot?
Internal electronics break down at temperature - classic intermittent failure pattern. Heat-soak test confirms.
How long does a CMP sensor last?
80,000-150,000 miles. Oil contamination from a leaking valve cover gasket shortens life.
Do I need a scan tool to relearn after replacement?
Some GM, Chrysler, and Hyundai vehicles - yes. Most others - no.