A faulty crankshaft sensor or a malfunction in the engine control unit causes your car to start poorly, cut out suddenly or not start at all. The engine control unit then no longer receives a reliable engine speed signal. As a result, injection and ignition fall outside control. This leads to multiple fault codes, loss of power and costly repairs. Read on to discover what causes these faults and how you can recognise them at an early stage.
what a crankshaft sensor does in your engine
The crankshaft sensor measures the position and speed of the crankshaft by registering the teeth on the reference wheel. A missing tooth serves as a reference point for the exact crankshaft position. The engine control unit then passes on this information to systems for injection timing, ignition timing and rev monitoring.
Without this signal, the engine control unit cannot control the engine correctly. If the signal fails or becomes unreliable, the engine cuts out or will not start at all. The rev counter on your instrument panel can also fail, because it uses the same sensor data.
Many owners of the Citroen Jumper report that a faulty crankshaft sensor causes problems mainly with a warm engine. After switching off, the engine is then reluctant to start again or will not start at all.
symptoms of a faulty crankshaft sensor
Typical complaints with a crankshaft sensor fault are starting problems, sudden stalling while driving and jerky running. The engine comes to a standstill without any prior warning. This sometimes happens in the middle of traffic. It happens because the engine control unit no longer receives an engine speed signal.
Other common symptoms are:
- Loss of power during acceleration
- Engine that will not restart after a warm run
- Engine stalling at idle
- Rough or hesitant running engine
- Engine management warning light on the dashboard
The complaints are variable. Sometimes the car runs fine for hours, only to cut out again for no obvious reason. This unpredictability makes it difficult to systematically trace the cause. The fault sometimes only occurs at specific temperatures or under certain conditions.
fault code P0335 and other diagnostic codes
The most frequently mentioned fault code with a crankshaft sensor problem is P0335. This code points to a fault in the crankshaft signal system or the crankshaft sensor circuit. However, this code does not automatically mean that the sensor itself is defective.
The problem sometimes lies in the wiring, connectors or a fault in a related system. A faulty engine control unit can itself cause multiple fault codes. Those codes then point to the crankshaft sensor, even though the sensor is still working fine. Systematically tracking down the cause is therefore complex. After replacing the sensor, the problem sometimes still remains.
When reading fault codes you will often see combinations of errors. A single P0335 code may occur together with codes for the camshaft sensor, fuel pressure sensor or other engine sensors. That combination helps the mechanic determine whether it is a circuit problem or an actually faulty sensor.
check wiring and connectors before you replace parts
First inspect the wiring and connections. Only then order a new crankshaft sensor. Corrosion, damaged cables or a loose connection cause the same symptoms as a faulty sensor. A poor contact disrupts the signal and leads to the same fault codes.
Check the following points:
- Unplug the crankshaft sensor connector and inspect the pins for corrosion or green deposits
- Check the wiring for visible cable breaks, especially at bends and mounting points
- Test the connector for a firm fit and replace it if you are in any doubt
- Make sure the cables are properly secured and do not rub against moving or hot engine parts
Clean all connections and fasten them securely. Then clear the fault codes. Next, test the car again. If the code does not return, you have avoided an expensive replacement through simple maintenance on wiring and connectors.
when to replace a faulty sensor
First make sure that wiring and connectors are in good order. If the fault code then returns, replacement of the crankshaft sensor is the next step. A sensor can fail internally. Causes include age, vibrations and excessive heat. This is particularly common on high-mileage engines.
Always replace the sensor with a part whose specifications match the original. When fitting, check the following points:
- The sensor is securely mounted
- The distance to the reference wheel is correct
An incorrect distance leads to a weak or intermittent signal. After replacement it is sometimes necessary to read or reset the engine control unit again. On some cars, temporarily disconnecting the battery helps, but a full diagnostic check with a professional scanner remains the safest way to confirm that the problem has been resolved.
problems with the engine control unit ECU
Sometimes all fault codes point to the crankshaft sensor, but after checking and replacing it, the problem turns out to be in the engine control unit itself. The ECU processes all sensor signals and controls the engine. A defect in the ECU means sensor signals are not interpreted correctly or that fault codes keep coming back.
Common complaints with a faulty ECU are multiple, contradictory fault codes and faults that do not disappear after replacing sensors. The engine may still start, but the engine management light then stays on. The cause remains unclear. In other cases the engine refuses to start at all, even after a reset or clearing fault codes.
costs and considerations when replacing the ECU
A replacement engine control unit often costs between 800 and 2000 euros, depending on make and model. On top of that come installation costs and programming work, because the new unit has to be matched to your specific car.
There are two alternatives. Option one is a reconditioned ECU. Option two is repair of the existing unit. A reconditioned unit often costs half the price of a new one, but does require the old ECU to be sent in. Repair is possible in some cases, but the defect must be traceable. Examples of repairable faults include a damaged printed circuit track or a faulty relay.
Not every garage has the equipment to programme an ECU correctly. Linking it to the immobiliser system also requires specialist equipment. Choose a specialist with experience in engine electronics to avoid ending up with a half-working car.
why faults come back after a reset
Clearing fault codes does not solve the underlying problem. If the cause of the fault has not been fixed, the fault code will return within a few kilometres or drive cycles. An ECU reset only sets the stored faults back to zero, but the sensor or circuit causing the problem remains defective.
The car may sometimes run better after clearing codes. This is because the engine control unit temporarily falls back on default values and allows the car to run in limp-home mode. As soon as the ECU resumes full analysis, it detects the fault again and the fault code returns.
With recurring faults, full diagnostic analysis is essential. This means systematically tracking down the cause. Pay attention to patterns and answer the following questions:
- Does the fault code only appear with a warm engine?
- Does the fault occur after long periods of driving?
- Does the code appear immediately after starting?
This information helps the mechanic to search specifically for an intermittent contact, a heat-sensitive sensor or a faulty component in the engine control unit itself.
practical tips for diagnosis and maintenance
Avoid unnecessary costs by first carrying out simple checks yourself. A good approach starts with reading fault codes using a professional scanner. Pay attention to the time at which the code was stored and whether it is active or historic.
Take photos of wiring and connectors before disconnecting components. This helps with refitting correctly and prevents mistakes during reassembly. Visually compare a new sensor with the old one to check that the part matches in terms of connectors and length.
Test the car immediately after it cuts out if the fault only occurs with a warm engine. Some fault codes are only visible briefly and disappear when the engine cools down. Have the diagnosis carried out while the engine is still warm so that the fault can be measured while it is active.
On the Carnews website you will find much more information on engine problems, technical diagnosis and maintenance advice for your car. Discover useful tips and practical articles that help you recognise car problems and save money.
frequently asked questions
The most common symptoms are difficult or no starting at all, especially with a warm engine, and suddenly cutting out while driving or idling. The engine may run irregularly or jerkily and deliver noticeably less power. Sometimes the rev signal also disappears or fluctuates strongly, which may indicate a dropping crankshaft signal. In practice, these complaints are often caused by the crankshaft sensor itself, but also by poor wiring, connectors or a fault in the engine control unit.
Faults in the crankshaft sensor disrupt the signal for crankshaft position and engine speed, so that injection and ignition are no longer correctly controlled and the engine starts poorly, runs irregularly or even cuts out. This leads to loss of power, juddering and sometimes complete engine stall, clearly worsening performance. Faults in the engine control unit (ECU) can cause the same complaints, because incorrect or missing control signals are sent to injectors and ignition. Both sensor and ECU problems greatly reduce reliability, because the car can cut out unexpectedly or refuse to start.
You can best diagnose faults with the crankshaft sensor or engine control unit by having them read out with a diagnostic tester, where fault codes (such as P0335) are checked and linked to complaints such as poor starting, stalling or irregular running. The mechanic then checks connectors and wiring for loose contacts, corrosion or broken cables to rule out a problem there. Next, it is often checked whether the crankshaft sensor actually sends a good signal to the engine control unit during starting. Understand the mechanic’s diagnosis as a combination of: fault code + measured signal + visual inspection of wiring and components.
Possible causes of faults in the crankshaft sensor and engine control unit include a defective sensor itself, damaged or corroded wiring/connectors, heat or vibration damage and internal faults in the ECU/ECM. Voltage problems (for example poor earth or battery voltage) can also cause fault codes and disturbances in the crankshaft signal. Solutions include checking and, where necessary, repairing or replacing wiring and connectors, measuring and possibly replacing the crankshaft sensor, and clearing fault codes after repair. For persistent problems, a specialist test or reconditioning/replacement of the engine control unit may be required.
Yes, regularly checking the connectors and wiring of the crankshaft sensor and engine control unit for corrosion, wear and loose contacts helps prevent many faults. Make sure cables do not chafe or kink and are properly secured, so that vibrations do not cause breaks or weak, fault-prone spots. During periodic maintenance, have the engine management system read out so that developing fault codes in the crankshaft signal or ECU are detected at an early stage. Keep the engine bay as clean and dry as possible around sensors and connectors to minimise problems caused by moisture and dirt.