Common Faults

Select a Vehicle Make to view a list of common faults

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Typical faults that may occur on different vehicles can include:

Poor idling - The idling of vehicle is maintained using the Idle Air Control Valve (IACV) and / or TCS (Throttle Control Switch). Some causes of poor idling can include an intermittent Ignition Coil, a bad O2 Sensor, a Vacuum Leak, Burnt Valves, Dirty or Burnt Plugs, a short on the Distributor Cap or Ignition Coil, or a bad Fuel Injector. A common cure for such an issue is a good emissions service at your local dealer.

Engine Not Cranking - this the where the engine will not turn over and start, and it may be accompanied by a warning light on your dash, such as an Engine Management light. Causes of this can include a faulty battery or starter motor (a battery can be simply tested by switching lights and indicators on and off), a lack of fuel or compression, or a broken timing belt or chain. Checks for these faults are usually the best port of call to begin with.

No Spark - this is when the engine cranks normally, but will not start because of a lack of spark, or it starts then stalls instantly. This may be caused by broken / loose / corroded wires from the pickup to the ignition module / PCM, a bad CKP / Crankshaft Position Sensor, or a bad rotor or distributor cap (cracks or carbon tracks that are allowing the spark to short to ground).

No fuel - This is where no fuel is able to reach the engine in order for it turnover and run. Common causes of this problem include a bad fuel pump / relay, Bad inertia fuel shut-off safety switch (Shuts off fuel in an accident, may have been tripped by a severe jolt, press button to reset), an open in wiring anywhere in fuel pump wiring circuit (power or ground), bad fuel, plugged / pinched fuel line or fuel filter, a leaky fuel pressure regulator, No power to Fuel Injectors (Due to faulty fuel injector relay, blown fuse, no input signal to PCM from crank position sensor or cam position sensor, or bad PCM driver circuit), Injectors should usually have power when key is on. PCM grounds other side of injector circuit to pulse the injectors. Also a major vacuum leak can cause bad fuel (An open EGR valve, disconnected vacuum hose, PCV valve, etc, can create a large vacuum leak and allow too much air to be sucked into the engine. This will make the air/fuel mixture too lean and make the engine hard to start. Engine will usually idle rough if it does start.

Engine cutting out when hot - this could be caused by a faulty MAF Sensor, bad Cam Position Sensor / Crankshaft Position Sensor, faulty Lambda Sensors, or a dirty Idle Air Control Valve (IACV).

Cold start problem - this is where a vehicle may struggle to start in cold weather conditions. It may be caused by an intermittent fault throttle body, a faulty idle valve or a fault coolant temperature sensor (CTS).

ABS / TCS lights staying on - this is where the Anti-lock Braking System or Traction control System warning lights stay lit on your instrument cluster. This can be caused by faulty wheel sensors (there is one on each wheel) or faulty wiring to the sensors from the ABS Pump/Modulator, a poor earth connection which can cause damage to the ABS Control unit, a burnt out motor (due to the modulator block seizing if the ABS system is not engaged often enough) which can overload the control unit, brake fluid changed incorrectly (causing non return valves to seize up and the modulator block to leak), or a faulty BAS (Brake Assist System). If the modulator block does seize up, it can have symptoms of the brake pedal moving away from your feet as though there is a leaky master cylinder. A side note is that some vehicles use the speedometer signal from the ABS ECU to operate the speedo, and also the Automatic transmission control module (common in Merecedes vehicles).

Engine ECU / Check Engine warning Light staying on - this problem occurs when the self check diagnosis of your vehicle fails for some reason, such as a problem with the emissions (see previous faults), an engine misfire, or a fault with the Engine Management ECU itself. As a rule, the light stays on as a result of a problem with the vehicle emissions.

Battery Charging Light - this light will come on if there is a fault with the battery charging system. This fault can be caused by a broken alternator belt or a fault with the alternator itself, dirty / bad battery connections, a poor earth connection, a faulty voltage regulator (typically on Mazda and Ford where the voltage regulator is built into the Engine Management ECU), or a fault with charging / overcharing protection relays. A fault with the protection relays may also cause further damage to the vehicle, in particular to any of the ECUs.

Diesel ECU fault - many modern diesel vehicles are fitted with a Diesel Control Unit alongside a high pressure pump, with later versions known as a Diesel HDI system. All diesel engine management systems are fitted with glow plug driver circuits, and most diesel engines are now fitted with turbo systems. A fault with any of these systems or modules can cause an ECU issue.

A good way of diagnosing a typical fault with most modern vehicles is to use a diagnostic tool and check for any fault codes. The fault codes given will often relate to a typical problem on the car.

Technical Data