What does it mean if a dashboard warning light on your car comes on ?

First check your owners manual to find out what the warning light refers to. Most warning lamps illuminate when the key is switched to ignition position and should go out when the vehicle engine is running.

Check Engine warning lamp

Illumination of the check engine or engine management warning ( MIL )  lamp indicates that the vehicle onboard computer (ECU , ECM ,PCM) control module has identified a fault generally related to the input sensors or output actuators controlling the efficient running of the engine and probably affecting exhaust emissions.
When the engine management light comes on the engine control system may revert to limp home or restricted mode.
This will invariably be noticed by the driver because of
poor acceleration, shuddering, restricted performance, or the engine will not rev above a certain rpm.

The check engine lamp illuminates when a value exceeds parameters programmed in to engine control memory map, this may be logged as a fault code related to a particular function or circuit. The engine control computer compares values from various sensors to calculate;

How hot or cold the engine is (intake air temperature,coolant temperaure)

Position of mechanical components (Crankshaft position sensor , Cam position sensor ), Driver demand (Throttle position sensor, brake pedal switch,clutch switch,etc)

Engine speed and load (airflow sensor, manifold absolute pressure,boost pressure)

Exhaust emissions (Lambda or Oxygen sensor fault code may occur)

Exhaust Gas Recirculation (EGR)

The value expected from each sensor is programmed into the engine control module and represented as a map.(Fuel map,Ignition map,etc)

An out of range value may represent a faulty component or wiring circuit.

Intermittent faults which occur during driving may be logged as an error fault code which cause the dashboard warning light to illuminate..

The next course of action in automotive  electrical diagnostics is to interrogate or scan the  vehicle computer  for stored fault codes using suitable diagnostic equipment such as a scan tool fault code reader.

Some vehicles (usually Japanese)  have a self diagnosis function allowing the the fault code to be read by counting the flashes of the check engine light and looking up the value in a reference table.

Most modern vehicles require diagnostic equipment with suitable software to interrogate the ecm for fault codes.

Following testing, a faulty wiring circuit may be repaired or component cleaned or replaced.

Our diagnostic equipment is capable of carrying out fault code reading and actuator testing to verify wiring circuits by operating the suspect component on a wide range of vehicle systems. The actuator test function can allow some components to be thoroughly cleaned which may save replacement cost.

Exhaust gas recirculation ( EGR ) system and Idle speed control ( ISC ) devices are particularly prone to failure due to build up of carbon and soot deposits.

EGR systems may cause the MIL warning lamp to illuminate intermittently and flag a P0400 EOBD fault code due to critical system malfunction.

A typical crankshaft position sensor used in a digital engine control system

Anti-Lock Braking System (ABS) warning lamp

Antilock braking operates by rapidly applying, and releasing the brake on each wheel just before it locks.

Faults within the Antilock braking system will cause the ABS light to illuminate. When the ABS system logs a fault this often causes the ABS to be disabled with affects on braking and steering control requiring increased pedal pressure as the system reverts to failsafe mode.

A permanently illuminated ABS warning lamp will result in MoT test failure as it represents a dangerous condition. Typical reasons for ABS light illumination are ; faulty wheel speed sensors,damaged or dirty rotor sensor ring,faulty modulator solenoids,abnormal battery voltage,system pressure faults,wiring faults.

Again first course of action is fault code reading using diagnostic equipment, possibly followed by individual component oscilloscope signal testing  or wiring checks then repair.

Anti-theft Immobiliser warning lamp

This may flash as the engine is cranked whilst the immobiliser is active.
Manufacturer or OEM immobiliser systems are often integrated into a vehicle on board computer system and may prevent starting, ignition, or fuelling circuits from operating.
Faults can sometimes occur if a receiver does not recognise the signal transmitted by a coded key or remote, or no transmission due to defective remote/discharged battery.
This type of system can rarely be bypassed other than by reprogramming.
Aftermarket alarm and immobiliser systems are installed as an added security measure by some owners. If these systems fail they can normally be removed or bypassed by an autoelectrician to enable vehicle operation.

You should check with your insurance company if an aftermarket alarm is to be removed as this may affect insurance policy validation.
There is more information about alarms & immobilisers on our vehicle security page.

Battery or Charging system warning lamp

This light indicates that either battery voltage is low or that the alternator is not generating current to recharge the battery.

Low battery voltage can be caused by a faulty battery, faulty alternator  / dynamo, loose / broken drive belt or a wiring fault.

Our mobile service can verify the condition of your vehicle battery using the midtronics battery tester, or check for alternator output ,and trace wiring faults which may be causing the alternator not to charge or the battery voltage to drop.

Systems on later vehicles may be computer controlled and require dedicated test equipment.

We also have the facility to bench test your alternator.

We can supply an extensive range of alternators starter motors and components and also provide unit repair and recondition facilities

link to products page

Bosch KTS equipment provides control unit diagnosis and multimeter functions

Airbag or SRS warning lamp

(Also known as SRS, Supplementary Restraint System, etc)

Illuminates if a fault is recognised by the airbag control module. Airbag deployment is controlled by sensors which react to impact above 20mph.

The SRS system self checks for wiring, sensor ,and actuator faults which may cause false triggering. If such a fault occur the system normally disables.

Faults can occur due to wiring, connections, impact sensors, control module faults, igniter defective, spiral cable defective, abnormal system voltage.

Systems have a self diagnosis function which logs fault codes when an error occurs and may then be diagnosed by interrogation procedure or suitable diagnostic equipment.

Coolant warning lamp

Low coolant level. Check for leaks as engine damage may occur due to loss of coolant. A low coolant level warning lamp may also illuminate if the wiring is chaffed and shorted to ground or the level sensor is faulty.

Temperature warning lamp

Indicates a problem with the engine cooling system which may cause engine damage. Possibilities are ; blocked radiator , faulty pressure cap, coolant leaks, combustion gas leak into cooling system, cooling fans not operatng due to motor ,switch,or wiring defects.

Diagnostic Training

Automotive diagnostics involves more than just plugging in a scan tool to read faults.

Diagnostic error codes provide a clue as to what the control system recognises as a fault but remember it is an electro mechanical system which is under repair.

 Motor trade experience combined with continuing professional development of staff through training courses aids vehicle system diagnosis & repair.