Your check engine light is on, the engine is running rough or stalling intermittently, and the codes point to either the crankshaft position sensor, the camshaft position sensor, or both. Both sensors are involved in ignition timing, and both can cause similar symptoms when they fail — which is exactly why distinguishing between them matters before you start replacing parts. Getting the diagnosis wrong means the problem comes back and you spend money on a sensor you did not need.
As a mechanic, I have diagnosed a lot of crank and cam sensor complaints, and the key to distinguishing between them is in the specific failure pattern — particularly whether the engine will start at all, whether the problem is worst during hot restart, and what the specific fault codes are saying about ignition timing correlation. I have had customers come in after replacing both sensors themselves without improvement, only to find out neither sensor was the actual problem — the reluctor wheel (the toothed ring the sensor reads) had a damaged tooth from a timing chain slap event. Understanding what each sensor does makes the diagnostic approach much clearer.
In this guide, I will walk you through what each sensor does, the specific symptoms that distinguish between them, the diagnostic tests that confirm which is failed, and what it costs to repair each.
Related troubleshooting: car stalls while driving and engine cranks but won’t start.
What Each Sensor Does And Why It Matters
The crankshaft position sensor (CKP) monitors the rotational position and speed of the crankshaft by reading a toothed reluctor wheel attached to the crankshaft. This data is the engine control module’s primary reference for ignition timing, fuel injection timing, and RPM. The CKP sensor is the master timing reference — without it, the ECM has no way to know where in the combustion cycle the engine is and cannot fire the ignition or injectors correctly.
The camshaft position sensor (CMP) monitors the position of the camshaft relative to the crankshaft. On a 4-stroke engine, the camshaft rotates at half the crankshaft speed, and its position tells the ECM which cylinder is on its compression stroke (ready to fire) versus its exhaust stroke (not ready to fire). The CMP sensor is used to synchronize sequential fuel injection — ensuring each injector fires the correct cylinder at the correct time — and for variable valve timing (VVT) systems that adjust cam timing for performance and efficiency.
One customer brought me a Ford F-150 that stalled intermittently, always at high engine temperatures and almost always immediately after a hot restart. It would start cold without issue, run normally, but if you shut it off when hot and tried to restart within a few minutes, it would crank and either not start or start and immediately stall. This specific pattern — hot restart failure — is the classic crankshaft position sensor failure signature on that engine. The sensor’s internal resistance changes with temperature, and the signal drops below the ECM’s minimum threshold when the sensor reaches a specific heat point. A P0316 code (misfire on startup) confirmed the pattern. CKP sensor replacement resolved it completely.
Comparing Symptoms: CKP vs CMP Sensor Failure
Here is how symptoms differ between the two failed sensors:
| Symptom | Points Toward CKP Sensor | Points Toward CMP Sensor |
|---|---|---|
| Engine won’t start at all | Yes — CKP provides primary timing reference | Less common — engine may run rough without CMP |
| Hot restart failure | Yes — common CKP failure pattern | Less common |
| Stalls then restarts normally after cooling | Yes — thermal CKP failure | Less common |
| Rough running, poor timing correlation | Yes — if intermittent signal | Yes — sequential injection affected |
| VVT system fault codes | No | Yes — CMP required for VVT operation |
| RPM signal missing from scan tool | Yes — CKP provides RPM data | No — CMP doesn’t generate RPM data |
Crankshaft Position Sensor Failure Characteristics
A failing CKP sensor produces symptoms that range from intermittent performance issues to complete no-start conditions depending on how far the failure has progressed. An intermittently failing CKP may produce occasional rough running, hesitation, or stalling that clears when the sensor cools down. A completely failed CKP results in a no-start condition — the engine cranks but will not fire because the ECM has no timing reference. The specific hot-restart failure pattern (cold start fine, hot restart fails or barely starts) is the most diagnostic symptom pointing to CKP sensor failure.
On the scan tool, a failed CKP sensor may show as zero RPM on the crankshaft sensor input even while the engine is cranking — a telltale sign that the sensor is not generating a signal. The specific fault codes P0335 (CKP sensor circuit range/performance) or P0336 (CKP sensor circuit intermittent) directly identify the sensor circuit as the problem area. A P0016 code (crankshaft and camshaft timing correlation) can indicate either sensor, but combined with a hot-restart pattern and a P0335 code, the CKP sensor is the clear suspect.
Camshaft Position Sensor Failure Characteristics
A failing CMP sensor produces different symptoms than a failing CKP. The engine can typically still start and run on the crankshaft timing reference alone — it may run rough, have reduced performance, and fall back to batch fuel injection rather than sequential injection, but it will usually run. Cold start difficulties and misfires during warmup can indicate a CMP issue. On vehicles with variable valve timing, a failed CMP sensor will disable the VVT system and trigger VVT-related codes alongside the CMP fault code.
The fault codes P0340 through P0344 (camshaft position sensor bank 1 or bank 2, various circuit faults) directly identify the CMP sensor circuit. On a V6 or V8 with two banks, the bank designation in the code identifies which CMP sensor is failing — important because replacing the wrong bank’s sensor wastes money and does not fix the problem.
How To Diagnose CKP vs CMP Failure Like A Pro
This is the same diagnostic process I use in the shop when either sensor code appears:
Step 1: Read All Codes And Identify The Pattern
I pull all stored and pending fault codes from the ECM and note the exact code numbers. P0335/P0336 codes identify the CKP circuit; P0340 through P0344 identify the CMP circuit. A P0016 timing correlation code combined with either of the above narrows which timing relationship is off. I look at the freeze frame data to understand under what conditions (engine temperature, RPM, load) the fault set — a code that set during a hot restart versus during cold idle is significant diagnostic information.
I also check the live data for both sensors while the engine is running. The CKP sensor should show a consistent RPM value matching the tachometer. The CMP sensor should show a camshaft position reading that changes with RPM. A sensor that shows erratic, jumping, or flat-zero values in live data while the engine is running is giving me direct evidence of a failing sensor or damaged reluctor wheel.
Step 2: Test Sensor Output And Inspect The Reluctor Wheel
With an oscilloscope or a graphing function on a professional scan tool, I can watch the waveform output from each sensor in real time. A healthy sensor produces a clean, consistent waveform as the toothed reluctor wheel passes by. A failing sensor shows a degraded or intermittent waveform — dropouts, reduced amplitude, or noise. A damaged reluctor wheel tooth shows up as a missing pulse in an otherwise clean waveform — a pattern that matches a reluctor wheel problem rather than a sensor problem and changes the repair approach entirely.
Reluctor wheel damage — typically a chipped or missing tooth — is less common than sensor failure but more expensive to repair because the reluctor wheel is often integral to the crankshaft or flywheel and requires significant disassembly to access. Distinguishing between sensor failure and reluctor wheel damage saves the customer from paying for a sensor replacement that will not fix a reluctor wheel problem.
Repair Costs
Professional Diagnosis
- Code scan and sensor circuit test: $75–$150
- Oscilloscope waveform analysis: Often included in diagnostic fee
Sensor Replacement Costs
- Crankshaft position sensor replacement: $150–$350
- Camshaft position sensor replacement: $120–$300
- CMP sensor on VVT actuator (more complex): $200–$500
- Reluctor wheel repair (if needed): $400–$1,200+ depending on location
How Urgent Are These Repairs?
Intermittent Stalling Or Rough Running: REPAIR IT SOON
Intermittent sensor failures can produce stalling conditions without warning at highway speeds. While the car may start and run normally most of the time, the risk of a sudden stall in traffic makes this an urgent repair.
- Avoid high-traffic or high-speed driving until repaired
- Repair within one week
- Do not ignore intermittent no-start codes — they will become permanent failures
No-Start Or Consistent Stalling: STOP DRIVING
A complete CKP sensor failure that produces a no-start condition is not drivable by definition. A sensor that is causing consistent stalling at speed is a safety issue.
- Have vehicle diagnosed and repaired before continuing to drive
- Tow to shop if no-start condition is occurring
How To Prevent Sensor Failures
Regular Maintenance
- Inspect sensor wiring harnesses during every major engine service for chafing or connector corrosion
- Change engine oil on schedule — sludge buildup can contaminate sensors mounted near oil passages
- Address any timing chain noise immediately — a worn chain can damage the reluctor wheel
Quality Parts And Service
- Use OEM or quality aftermarket sensors — cheap sensors often have poor-quality Hall effect elements that fail prematurely
- Clean the sensor mounting bore and check for metal debris before installing new sensors
- Verify the correct air gap specification is met if adjustable sensor mounting is involved
FAQ: Crankshaft vs Camshaft Sensor Questions Answered
Can a bad camshaft sensor prevent the car from starting?
On most modern vehicles, a failed camshaft sensor alone will not prevent the engine from starting — the ECM can use the crankshaft sensor as the primary timing reference and will fall back to a limp-home mode with batch injection rather than sequential injection. The engine may run rough or with reduced performance, but it will typically start. A failed crankshaft sensor is much more likely to produce a complete no-start.
Can I drive with a bad crankshaft or camshaft sensor?
A partially failing CKP sensor that causes intermittent stalling is unsafe to drive on at highway speeds because a stall can occur without warning. A completely failed CKP sensor that produces a no-start is obviously undrivable. A partially failing CMP sensor on a vehicle without VVT may produce rough running but allow continued driving for a short period — though the performance degradation and potential for sudden stalling warrants prompt repair.
What does the waveform tell me that a code doesn’t?
Fault codes tell you which circuit has a problem but not the specific failure mode within that circuit. The waveform shows whether the sensor is generating a clean signal with proper amplitude and consistency, whether there are dropouts (signal disappears for specific pulses — indicating a chipped reluctor wheel tooth), or whether the signal is degraded throughout (indicating a failing sensor element or weak magnet). A missing tooth on a reluctor wheel produces a code-identical fault to a failing sensor but requires a completely different repair.
Wrapping It Up
The crankshaft position sensor provides the primary timing reference — its failure most directly causes no-start conditions and hot-restart failures. The camshaft position sensor synchronizes sequential injection and VVT operation — its failure typically produces rough running and VVT faults but usually allows the engine to continue starting. Reading the specific fault codes and noting the exact symptom conditions (hot or cold, start or running) is the fastest path to the correct diagnosis.
Mechanic’s Tip: Before replacing either sensor, check the sensor connector for corrosion and the wiring harness for chafing near hot components or sharp edges. I have found CKP and CMP sensor codes that were caused by nothing more than a corroded terminal in the connector that was intermittently breaking contact. A five-minute connector inspection and cleaning has resolved sensor fault codes without replacing any sensors on more occasions than I can count.
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