Car Won't Start? How to Test a Crankshaft Position Sensor (4 Tests)

Engine cranks but won't start? Stalls when hot, jerks or won't rev above about 3,000 RPM? The culprit can be a tiny part held by just one bolt: the crankshaft position sensor (CKP sensor). This guide explains, with a 3D engine model, how it works, where to find it and how to test it properly before you replace it without need.

Video: Car Won't Start? How to Test a Crankshaft Position Sensor – 4 Tests (3D Animation), 5:27 min, by Auto Oprema. The video has English and Serbian audio (choose the audio track in the ⚙ settings) and subtitles.

The video is published on 12 November 2026. Until then the player shows that it is not yet available; the guide below is already complete.

Quick answer: how do you know the crankshaft sensor is bad?

If the sensor fails completely, the engine cranks but won't start, and live data shows 0 RPM while cranking.
Intermittent faults show up when the engine is hot or at higher RPM: loss of power, no revs above about 3,000 RPM, jerking or stalling – often without any fault code.
Test it with live data, a multimeter (from the back of the connector), a wiring check to the ECU and by tapping and warming the sensor – the best tool is an oscilloscope.

How the crankshaft position sensor works

On the crankshaft there is a toothed wheel, called a reluctor or trigger wheel. It usually has 60 teeth with 2 missing. As the crankshaft turns, the sensor reads every tooth passing by. From the speed of the teeth the engine computer knows the RPM, and from the gap it knows exactly where the pistons are. That is how it decides when to inject fuel – at top dead centre or slightly before or after. Without this signal the computer does not know when to inject, and the engine won't run.

Where is the crankshaft sensor?

At the front – on many engines, like the 1.6 HDi, right next to the crankshaft pulley, behind the accessory belt.
At the back – near the flywheel and the gearbox. Sometimes it is very hard to reach: on some 4x4 models you have to remove the wheel and the inner wheel liner, and an inspection camera helps.

The sensor is held by a single bolt. Pull it out gently, turning it slightly.

Two types of crankshaft sensors

Type	Wires	Signal
Inductive	2	Generates its own voltage: an alternating sine wave that gets stronger as the engine speeds up.
Hall or magnetoresistive (more common)	3: ground, constant 5 V reference, signal in the middle	A clean square wave switching between 0 and 5 V.

Symptoms of a bad crankshaft position sensor

Complete failure: the engine cranks but won't start.
Intermittent fault (worse): the sensor works when cold, but when the engine heats up or at higher RPM the signal goes out of range. The computer switches to safe mode: the engine loses power, won't go over about 3,000 RPM, jerks or even stalls.
Often no fault code is stored at all.

How to test a crankshaft position sensor: 4 tests

Test 1: Live data with a diagnostic tool

Open engine speed and crank the engine. Even while cranking you must see RPM, around 150–250 RPM. If it stays at 0, the computer gets no crankshaft signal and won't inject fuel. (Unplug the sensor and crank: RPM stays at 0. Plug it back in and the engine starts right away.) When the engine runs, rev it up – the RPM must rise and fall smoothly, without drops.

Test 2: The connector, with a multimeter

Never push the probes into the front of the connector, where the sensor pins go in – you will spread the terminals and create a bad contact that is very hard to find. Always probe from the back. With the ignition on:

Ground: close to zero, just a few millivolts.
Reference pin: about 5 V (4.98 V in the video).
Hall sensor: pass a piece of metal past the tip – the signal switches between 5 V and 0.

With the engine running, a multimeter is too slow for this signal – that is a job for the oscilloscope.

Test 3: The wiring to the engine computer

Disconnect the ECU connector and use a wiring diagram to find the sensor pins. Back-probe the pins and measure each wire from the ECU plug to the sensor plug. Every wire should show almost zero ohms, around 0.1–0.2 Ω. While measuring, wiggle the harness, especially in hot areas near the engine. If the value jumps, you have found a broken wire.

Test 4: Hunting intermittent faults

Connect the sensor and watch the signal. Lightly tap the sensor body with the handle of a screwdriver. If the signal drops out, the sensor has an internal crack.
Heat also reveals faults: warm the sensor, or even the engine computer, with a hair dryer – warm, not hot. If the engine stalls only when it is warm, you have found the culprit.
The best tool is an oscilloscope: a good sensor shows clean, even pulses with a clear gap for the missing teeth. Missing or distorted pulses mean trouble.

Summary

A bad crankshaft sensor can mean no start, stalling when hot or limp mode. Don't replace it without testing: check RPM in live data while cranking, test the connector from the back, check the wiring to the ECU and hunt intermittent faults with tapping, heat and an oscilloscope.

Frequently asked questions

What are the symptoms of a bad crankshaft position sensor?

The engine cranks but won't start, or it stalls when hot, jerks, loses power and won't rev above about 3,000 RPM. Often no fault code is stored.

How can I test a crankshaft sensor without removing it?

Check engine speed in live data while cranking – you must see about 150–250 RPM. Then check the connector from the back with a multimeter: ground near 0 V and a reference of about 5 V.

Why does the engine stall only when it is hot?

A failing sensor can work when cold, but when it heats up the signal goes out of range. Warming the sensor with a hair dryer can reveal this fault.

Can a bad crankshaft sensor cause limp mode?

Yes. When the signal goes out of range the computer switches to safe mode: the engine loses power and won't go over about 3,000 RPM.

Where is the crankshaft position sensor located?

Either at the front next to the crankshaft pulley (for example on the 1.6 HDi) or at the back near the flywheel and gearbox.

What is the difference between an inductive and a Hall crankshaft sensor?

An inductive sensor has 2 wires and generates its own sine-wave voltage. A Hall or magnetoresistive sensor has 3 wires (ground, 5 V reference, signal) and gives a square wave between 0 and 5 V.

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