What sensor that can cause a car to crank but not start?

The crankshaft position sensor (CKP) is the sensor most likely to cause a car to crank but not start when it fails.

In modern vehicles, the CKP provides the engine control unit (ECU) with timing data essential for spark and fuel delivery. If the CKP signal is missing or erratic, the engine may turn over but fail to start. While other sensors can contribute to a no-start condition, CKP failure is the classic spark-fuel timing culprit you’ll hear about from technicians.

Why the crankshaft position sensor matters

The CKP tells the ECU the exact position and speed of the crankshaft, allowing the engine to ignite sparks and inject fuel at the proper moments. When the CKP signal is lost or out of range, the ECU may refuse to fire the injectors or spark, resulting in a crank-but-no-start scenario. Some vehicles may still attempt to start if the signal is intermittently available, but reliability will be poor.

Key indicators that CKP or related circuitry could be at fault include the following:

• Engine cranks but fails to start or runs for a moment and dies


• Intermittent no-start conditions, often worsening with engine heat


• Check Engine Light with codes such as P0335, P0336 (CKP circuit/misfire range or performance)

These symptoms guide technician testing toward the crankshaft position sensor and its wiring, rather than toward purely mechanical issues like fuel pump failure.

How to diagnose a CKP-related no-start

Before confirming a CKP fault, use a systematic approach to verify whether the signal is present and within specification. The following steps help distinguish CKP issues from other problems such as CMP faults or immobilizer faults.

• Scan the vehicle for trouble codes (P0335, P0336, or related CKP/CMP codes) and note any patterns


• Inspect the CKP sensor, its wiring harness, and connectors for damage, corrosion, or loose connections


• Check the reluctor ring and timing components for physical damage or misalignment


• Test the CKP signal with a scan tool or oscilloscope while cranking to confirm a clean, consistent waveform


• Rule out related sensors (CMP) and immobilizer-related issues if CKP appears functional

If the CKP signal is absent, erratic, or out of range, replacing the sensor often resolves the no-start condition. In some cases, wiring repairs or a faulty reluctor ring are the actual culprits rather than the sensor itself.

Other sensors and systems that can keep a car from starting

Camshaft position sensor (CMP)

The CMP works alongside the CKP to time ignition and fuel. A failing CMP can also cause a no-start or misfire, and some vehicles will display CMP-specific codes (such as P0340 and related variations). In certain designs, a CMP failure might still allow a start, but with poor timing and performance, or with intermittent starts.

Immobilizer and key transponder

Even with a healthy CKP signal, an anti-theft immobilizer system can prevent starting if the key transponder isn’t recognized by the vehicle’s security ECU. In such cases, the starter may spin, but fuel and spark are blocked until the immobilizer issue is resolved. Symptoms can include a security light staying on or flashing and no-start conditions even when other sensors test fine.

Summary

For most no-start, cranking-but-not-start scenarios, the crankshaft position sensor is the primary suspect. Start diagnostics by pulling codes related to CKP, inspecting the sensor and wiring, and verifying the CKP signal with appropriate equipment. Don’t overlook the camshaft position sensor and immobilizer system, which can mimic CKP problems in some vehicles. If in doubt, a professional technician can perform definitive electrical tests and, if needed, sensor replacement.

Bottom line

The crankshaft position sensor is the sensor most closely tied to cranking without starting. A failing CKP disrupts timing information the ECU needs to spark and fuel the engine, making it a common and often reparable culprit in modern cars.

Why would a car crank over but not start?

A car that cranks but won't start is likely experiencing a problem with either the fuel system, the ignition system, or engine compression. Common causes include a faulty fuel pump, clogged fuel filter, bad spark plugs, or a bad crankshaft position sensor. 
This video explains common causes for a car to crank but not start: 1mEasyAutoFixYouTube · Sep 9, 2023
Possible solutions and diagnoses
1. Fuel system issues

• Empty fuel tank: First, check the fuel gauge. Sometimes, a car on a hill with low fuel won't start. 
• Fuel pump: Listen for a humming sound from the back of the car when you turn the key to the "on" position. If you don't hear it, the fuel pump or its relay could be the issue. 
• Clogged fuel filter: A clogged filter can prevent fuel from reaching the engine, even if the pump is working. 
• Using starting fluid: Spray a small amount of starting fluid into the air intake and try to start the car. If it starts briefly and then dies, it points to a fuel delivery problem. 

2. Ignition system issues

• Bad spark plugs: Worn-out spark plugs can't ignite the fuel-air mixture, causing the engine to fail to start. 
• Faulty crankshaft position sensor: This sensor is crucial for telling the engine's computer when to create a spark. If it fails, the engine won't start. 
• Other ignition parts: Damaged or faulty spark plug wires, coils, or a distributor cap (in older cars) can also prevent a spark. 

This video shows how to test for spark and diagnose ignition problems: 1mScotty KilmerYouTube · Mar 30, 2010
3. Mechanical problems

• Low or no engine compression: This can be caused by a worn-out timing belt or a blown head gasket, which are more significant issues. 
• Faulty anti-theft system: An immobilizer or anti-theft system can prevent the car from starting. 

Other checks

• Battery: A weak battery can cause slow cranking. However, if the car cranks at a normal speed but still won't start, the battery is likely not the main issue. 
• Check for trouble codes: Use an OBD-II scanner to check for any trouble codes, which can provide more specific diagnostic information. 

What sensors can cause crank no start?

Your vehicle's crankshaft position sensor plays a key role in keeping your engine running. It tells the engine control unit (ECU) when to fire the spark plugs and inject fuel. If this sensor stops working, your car may crank but won't start because the engine's timing is off.

What happens when the crank sensor goes bad?

When a crankshaft position sensor goes bad, the vehicle's computer receives incorrect information about engine speed and position, which can cause a variety of issues, including the engine stalling, difficulty starting, rough idling, and poor acceleration. A faulty sensor can also trigger the check engine light. In severe cases, it can lead to the engine completely refusing to start, as the computer won't know when to time the fuel injection and spark for combustion.
 

• Engine stalling: The engine may stall unexpectedly while driving, at idle, or during acceleration. 
• Hard or no start: The engine may crank longer than usual before starting, or it may not start at all, especially when the engine is hot. 
• Rough idling and misfires: The engine may shake or run roughly at idle because the incorrect timing for fuel and spark causes misfires. 
• Poor acceleration: The vehicle may feel sluggish and not accelerate smoothly due to the engine's performance being affected by improper fuel and spark timing. 
• Check Engine Light: A malfunctioning sensor will often trigger the check engine light on the dashboard. 
• Decreased fuel efficiency: The engine's fuel injection timing can be disrupted, leading to worse gas mileage. 
• Irregular RPMs: The tachometer may show erratic readings, with sudden jumps or drops. 

Which sensor can stop a car from starting?

The most common sensor to cause a car not to start is the crankshaft position sensor, as it is essential for the engine's computer to know when to fire the spark plugs and inject fuel. Other sensors that can prevent a start include the camshaft position sensor, mass air flow (MAF) sensor, and oil pressure sensor. 
You can watch this video to learn about the most common causes of a car that cranks but won't start: 1mEasyAutoFixYouTube · Sep 9, 2023
Common sensors that prevent a car from starting

• Crankshaft Position Sensor (CPS): This is a very common cause. It tells the engine's computer the position and speed of the crankshaft. If this sensor fails, the computer doesn't know when to send the spark to the cylinders, and the engine won't start. 
• Camshaft Position Sensor: This sensor works with the crankshaft position sensor to tell the computer the position of the camshafts. This is crucial for timing both the spark and fuel injection. A bad one can prevent the engine from starting. 
• Mass Air Flow (MAF) Sensor: This sensor measures the amount of air entering the engine. If it gives a faulty reading, the computer may not send enough fuel, and the air-fuel mixture will be incorrect for combustion, leading to a no-start condition. 
• Oil Pressure Sensor: In some vehicles, a failing oil pressure sensor can trigger a fail-safe mode that prevents the engine from starting. This is a protective measure because the computer believes there isn't enough oil pressure to run the engine safely. 
• Fuel Pressure Sensor: This sensor monitors the pressure in the fuel rail and relays that data to the engine's computer. If it malfunctions, the computer may not be able to regulate the fuel pump, which can lead to a no-start situation.