Dashboard warning lamp

We DO NOT recommend vehicle operation is continued with a warning lamp or warning message active – It is a warning for a reason !

First check your owners manual to find out what the warning light refers to. Operation sequence of most engine system related dashboard  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: The engine control fault code light

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  related to the input sensors or output actuators controlling the efficient running of the engine and probably affecting exhaust gas emissions. Vehicles with diesel engines may also use the glow plug pre heat warning lamp to indicate engine management information.
If the dashboard engine management light comes on with the engine operating  a fault code has almost certainly been logged by the engine control system, and consequently the control strategy  may revert to limp home or restricted power mode. Engine management faults in some cases  may  prevent the engine  from  running by software function..
Limp home mode will usually be noticed by the driver because of  poor acceleration, shuddering, restricted performance, or the engine will not rev above a certain rpm.
What causes the engine management lamp to light up ? Can you turn the MIL light off ?
The value expected from each engine sensor is programmed into the engine control module and represented as a map or lookup table. (Fuel map, Ignition map, etc)
An out of range value may represent a faulty component or wiring circuit. The check engine lamp illuminates when a value exceeds parameters programmed in to engine control memory map by the vehicle  manufacturer, this fault may be logged as either an EOBD (European On Board Diagnostics) fault code or manufacturer specific fault code related to a particular function or circuit. The nature of the fault may be electrical or mechanical. Diagnostic equipment will reset the control system computer and turn the warning light off , however if a fault is still present the check light will often  illuminate again once a driving cycle has been completed until the fault has been rectified. In some cases several error codes may be present.

The engine control computer module compares values from various sensors to calculate;

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

Random disconnecting of electrical connectors in the engine bay may also cause  fault codes to be logged.

Retrieval of fault codes

Most modern vehicles require diagnostic equipment with dedicated software to interrogate the ecm for stored fault codes.
Automotive  system electrical diagnostics involves an interrogation  scan of the  vehicle computer  for stored fault codes using suitable diagnostic equipment such as a scan tool fault code reader.

Some older 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.

Test and Repair

Following testing, a faulty wiring circuit may be repaired or component cleaned or replaced.
Diagnostic Equipment Functions
Our diagnostic equipment is capable of

Sensors, Actuators , and Common Faults

Exhaust gas recirculation ( EGR ) system functions to reduce combustion temperature and oxides of Nitrogen ( Note : not Nitrous oxide !) EGR and Idle speed control ( ISC ) devices are particularly prone to failure due to build up of carbon and soot deposits which can restrict the air inlet system of the engine.

EGR systems may cause the MIL warning lamp to illuminate intermittently and flag a P0400 EOBD fault code due to "critical system malfunction" as described on certain diagnostic equipment !

Mechanical  position sensors :

Cam position sensor : used to identify cylinder phasing.

Crankshaft (CKP)position sensor : Identification of engine crankshaft position and calculation of engine speed and combustion forces. Frequency and amplitude signal is proportional to engine rotational speed.

Typical failure : Signal errors can occur due to failed sensor , phonic wheel damage, or wiring connection failure.

Throttle position sensor (TPS) / Accelerator Position sensor (APP)
function:Driver demand signal. TPS  failure may occur due to wear of potentiometer track causing flat spots and hesitation when accelerating.

Brake Pedal Position & Clutch Pedal Position sensors used to reduce to stabilise idle speed and fuel delivery quantity when  engine is off load . Faults may occur due to worn out switches resulting in false (implausible) signals.

Mass Air Flow (MAF) sensor measures volume of air entering air intake , Air Mass Meter measures weight of air entering intake tract. Engine load sensor which can effect fuel metering (petrol engine) or EGR position (diesel engines) .

Intake Air Temperature Sensor may be incorporated with air mass meter , with MAP sensor (T-MAP) or seperate.

Manifold Absolute Pressure (MAP) sensor measures engine load by vacuum (petrol engines) or boost pressure (turbocharged engines) at intake manifold. Split or blocked pipes can effect operation. MAP signal can influence fuel control, EGR, and turbo boost.

Temperature measurements may be signalled by either coolant temperature sensor or cylinder head temperature sensor. Temperature signal is processed to compensate engine smooth running by control of fuel delivery  egr position, & diesel pre heat function.

Fuel Delivery System :  Low and high pressure fuel injection pumps, pressure regulator, fuel injectors. Prone to faults caused by fuel contamination , and general wear.

Boost pressure control : turbocharger waste gate position and hence boost pressure control is often actuated by solenoids controlling vacuum circuits. Common faults can include loss of vacuum due to vacuum pump wear, split or blocked vacuum pipes, electrical circuit faults with control, solenoid or wiring.

 Lambda Sensor (fitted to Petrol & Gas fuel systems)
Measures oxygen content of exhaust gas and is used as a feedback signal to control fuel quantity delivery. Can deteriorate due to age and contamination. Lambda sensor fault codes may also relate to problems other than the sensor itself. Lambda sensor operation can is affected by sensor temperature .Lambda sensor feedback signal is only used when engine operating temperature is sufficient to enable closed loop control status.
A second lambda sensor may be fitted in the exhaust system after the catalytic convertor to monitor condition of the catalyst.

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.

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.

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 system manufacturer training courses aids vehicle system diagnosis & repair.