What Happens If the Crankshaft Sensor Goes Bad?

When a crankshaft position sensor goes bad, the engine can misfire, run roughly, become hard or impossible to start, and may stall unexpectedly—often triggering a check-engine light and storing diagnostic trouble codes. In severe cases, the vehicle will crank but never actually start because the engine computer no longer knows where the crankshaft is in its rotation.

Why the Crankshaft Sensor Matters

The crankshaft position sensor (often called “CKP sensor”) is a critical engine management component that tells the engine control unit (ECU) the exact position and speed of the crankshaft. Without this data, modern engines cannot accurately control fuel injection and ignition timing, which are essential for combustion.

Key Symptoms of a Failing Crankshaft Sensor

The following bullet points highlight the main warning signs most drivers notice when a crankshaft sensor starts to fail, ranging from subtle performance issues to complete engine shutdown.

• Intermittent no-start condition: The engine cranks (starter motor spins) but does not fire, especially when hot or after a short drive.
• Engine stalling while driving: The engine may suddenly shut off at idle, during acceleration, or at highway speed, sometimes restarting after a cool-down.
• Rough idle and misfires: The engine may shake, hesitate, or feel uneven at idle or under load because timing signals are erratic.
• Loss of power and poor acceleration: The car may feel sluggish, with delayed throttle response or surging during acceleration.
• Check-engine light (CEL): Modern vehicles typically log trouble codes such as P0335–P0339, which indicate issues with the crankshaft sensor circuit or signal.
• Reduced fuel economy: Incorrect timing can cause incomplete combustion, wasting fuel and potentially increasing emissions.
• Hard starting when hot: Heat-related sensor failure is common; the engine might start cold, then refuse to start after being driven and parked briefly.
• No tachometer movement while cranking: On many vehicles, a dead crank signal means the tach needle stays at zero during cranking.

Taken together, these symptoms strongly suggest that the ECU is losing reliable crankshaft position information, forcing the engine either to run poorly or to shut down altogether as a self-protection measure.

What the Crankshaft Sensor Actually Does

How It Works

The crankshaft position sensor monitors a toothed wheel (reluctor ring) mounted on the crankshaft. As the crank rotates, the teeth pass by the magnetic or Hall-effect sensor, generating a digital signal. The ECU uses this signal to determine:

The following list explains the core data points the ECU derives from the crankshaft sensor signal.

• Crankshaft rotational speed (RPM): Essential for fuel delivery, ignition timing, and idle control.
• Crankshaft angle/position: Determines when each cylinder is at top dead center (TDC) for firing.
• Synchronization with camshaft sensor: In engines with variable valve timing and sequential fuel injection, the ECU compares crank and cam signals to time both fuel and spark with high precision.

Without a clean, consistent signal from the crankshaft sensor, the ECU essentially loses its primary timing reference and cannot coordinate combustion events properly.

Crankshaft Sensor vs. Camshaft Sensor

Many engines use both crankshaft and camshaft position sensors. The crankshaft sensor primarily controls core timing and RPM, while the camshaft sensor helps with cylinder identification and valve timing. In some vehicles, the ECU can “limp” on camshaft data if the crank sensor signal is weak, but in most designs, a total loss of crank signal will shut the engine down.

Driving With a Bad Crankshaft Sensor: Risks and Realities

The following list outlines the key risks drivers face if they continue operating a vehicle with a known or suspected crankshaft sensor problem.

• Sudden stalling in traffic: The engine may cut out without warning, creating a safety hazard in intersections, on highways, or during overtaking.
• Unreliable restarts: The vehicle might restart after cooling down, only to stall again, leaving the driver stranded unpredictably.
• Potential catalyst damage: Misfires and incomplete combustion can send unburned fuel into the catalytic converter, risking overheating and costly damage.
• Inability to pass emissions tests: If misfires and timing issues persist, emission levels may exceed legal thresholds.
• Increased wear on starter and battery: Repeated extended cranking to get the engine started puts extra stress on starting components.

Because of these risks, most technicians recommend addressing crankshaft sensor issues promptly rather than continuing to drive until total failure occurs.

Common Causes of Crankshaft Sensor Failure

The following list summarizes the typical reasons a crankshaft sensor or its signal can fail, beyond simple age-related wear.

• Heat and vibration: Sensors mounted near the engine block face continuous thermal cycling and vibration, which can crack internal components or solder joints.
• Oil or coolant contamination: Leaking seals can allow fluids to enter the sensor connector or housing, degrading the signal.
• Wiring and connector issues: Corroded terminals, broken wires, or loose connectors can interrupt or distort the signal.
• Damaged reluctor ring: Missing, bent, or dirty teeth on the tone wheel can cause erratic readings or false triggers.
• Aftermarket or poor-quality replacement parts: Low-grade sensors may fail prematurely or produce marginal signals that cause intermittent symptoms.
• Physical impact: Previous engine work or road debris can damage exposed sensors or wiring.

Identifying the underlying cause, not just replacing the sensor, helps prevent repeat failures and ensures a more reliable repair.

How Mechanics Diagnose a Bad Crankshaft Sensor

Warning Lights and Trouble Codes

Most drivers first notice a check-engine light. Scanning the vehicle’s onboard diagnostics typically reveals codes such as:

The following list shows the OBD-II codes most often associated with crankshaft sensor faults.

• P0335: Crankshaft Position Sensor “A” Circuit – general fault in the sensor or wiring.
• P0336: Crankshaft Position Sensor Range/Performance – signal not matching expected patterns.
• P0337: Crankshaft Position Sensor Circuit Low Input.
• P0338: Crankshaft Position Sensor Circuit High Input.
• P0339: Crankshaft Position Sensor Circuit Intermittent.

These codes point technicians toward the crankshaft sensor circuit but do not by themselves confirm whether the sensor, wiring, or tone wheel is at fault.

Testing Procedures

Professionals typically combine live data, electrical tests, and visual inspection to confirm the diagnosis.

The following list outlines standard diagnostic steps used in workshops.

• Live data monitoring: Using a scan tool, technicians watch RPM and crank signal data while cranking and running; a flatline RPM during cranking is a red flag.
• Oscilloscope analysis: On more detailed jobs, an oscilloscope is used to view the exact waveform from the sensor, revealing missing or distorted pulses.
• Resistance and continuity checks: The sensor’s internal resistance is measured and wiring continuity is checked against manufacturer specifications.
• Connector and wiring inspection: Technicians look for corrosion, loose pins, chafed wires, or fluid intrusion at connectors.
• Reluctor ring inspection: Where accessible, the tone wheel is checked for cracks, missing teeth, or heavy debris buildup.

By combining electronic testing with a physical inspection, mechanics can distinguish between a genuinely failed sensor and a wiring or mechanical issue that mimics sensor failure.

Repair, Replacement, and Costs

What the Repair Involves

On many modern vehicles, replacing the crankshaft sensor is straightforward, but access varies significantly by engine layout.

The list below describes typical aspects of the replacement process and what affects cost and complexity.

• Location-dependent labor: Some sensors are easily accessible near the bottom or side of the engine; others require removing engine mounts, covers, or other components.
• Sensor calibration or relearn procedure: Many vehicles require a “crank relearn” using a scan tool so the ECU can adapt to the new sensor’s signal profile.
• Wiring repair if needed: Damaged or corroded wiring and connectors may need splicing, replacement, or cleaning.
• Reluctor ring repair or replacement: If the tone wheel is damaged (especially on some European and performance vehicles), the repair may require more extensive disassembly.
• Part quality choice: OEM or high-quality aftermarket sensors generally provide more reliable, consistent performance than very cheap alternatives.

Overall, the job can range from a relatively quick sensor swap to a more involved operation if access is poor or other components are affected.

Typical Price Ranges (as of 2024)

Costs vary by make, model, and region, but broad ranges are relatively consistent.

The following list gives ballpark cost figures many owners can expect for a crankshaft sensor repair.

• Sensor part cost: Approximately US$30 to US$200, depending on vehicle brand and whether OEM or aftermarket.
• Labor cost: Roughly 0.5 to 3.0 hours of labor, commonly US$75 to US$400+ depending on labor rates and difficulty of access.
• Total typical repair: Around US$150 to US$600 for most mainstream vehicles; significantly more if the reluctor ring or related components are damaged.
• Additional diagnostic fees: Some shops charge separate diagnostic time, especially for intermittent issues that require extended testing.

For many vehicles, fixing a failing crankshaft sensor is a mid-range repair cost that prevents far more expensive damage to the catalytic converter and other components down the line.

Prevention and Maintenance Tips

The following list highlights practical ways to reduce the likelihood of crankshaft sensor problems and to catch issues early.

• Address oil and coolant leaks promptly: Prevent fluids from contaminating the sensor and its connector.
• Avoid high-pressure washing around engine electronics: Excessive water can enter connectors and cause corrosion or shorts.
• Schedule regular inspections: During routine service, ask for a quick check of exposed wiring harnesses and connectors for wear.
• Use quality replacement parts: When replacement is needed, select reputable brands or OEM sensors for better reliability.
• Pay attention to early symptoms: Do not ignore brief stalls, intermittent no-starts, or unexplained misfires—early diagnosis is usually cheaper and safer.

While there is no formal “service interval” for crankshaft sensors, good general engine maintenance and early response to small issues help extend sensor life and avoid sudden breakdowns.

Bottom Line: What Drivers Should Do

If a crankshaft sensor goes bad, the vehicle may run poorly, stall, or stop starting altogether. Because the sensor is central to engine timing, ignoring the early warning signs can quickly lead to dangerous driving situations and more expensive repairs.

Drivers who notice sudden stalling, intermittent no-starts, or a check-engine light with crank-related codes should have the vehicle scanned and inspected as soon as possible. With proper diagnosis and a quality replacement, most crankshaft sensor issues can be resolved quickly, restoring reliable performance and reducing the risk of being stranded.

Summary

A failing crankshaft position sensor disrupts the engine’s ability to time fuel injection and spark, leading to rough running, misfires, hard or no-start conditions, and unexpected stalling. The issue is often accompanied by trouble codes such as P0335–P0339 and a check-engine light. Causes range from heat and vibration damage to wiring faults and damaged reluctor rings. Diagnosis typically involves scanning for codes, checking live data, inspecting wiring, and sometimes using an oscilloscope. Repair usually means replacing the sensor—sometimes along with wiring or a tone wheel—with total costs commonly in the US$150–US$600 range. Addressing symptoms early and maintaining the engine’s overall health significantly reduces safety risks and the likelihood of a roadside breakdown.