Controller Area Network BUS (CANbus), CAN FD

The Controller Area Bus Network or CAN network allows two way data communication between vehicle electronic computer modules.

Vehicle computers may pass messages on a bus system. In this case the bus system refers to the physical wiring cables between computer controllers. Messages are passed along cables as data often in the form of electrical pulses. The CANbus system has two voltage signal levels: CAN High (CAN-H) and CAN Low (CAN-L). Seperate cables CAN-H and CAN-L form a twisted pair of cables.

CANbus signalling faults occur for many reasons :
Controllers fail,
Controllers are not configured or matched. CAN network configuration coding mismatch introduces faults where one module affects other modules on the vehicle network.
cables become damaged or break causing open circuits or short circuit,
Cables induce spurious electrical currents,
Data messages are not synchronised so crash.

CAN FD refers to a faster system of message communication using Flexible Data.

Vehicle Network Configuration

Vehicle Configuration Test and System (VCATS) engineering is a function often performed by factory auto electricians and engineers at vehicle manufacture. Fitting or removal of computer control modules generally requires VCATS resets of some description to be performed for the vehicle to work properly. Power disconnection / reconnection may require module reset or programming procedures using diagnostic test equipment. Vehicle network communications can send and receive data on different types of systems either optical, wired or wireless.

Instrument Cluster

The Instrument cluster provides guages and warning lamps giving the driver a visual and audio signal of vehicle operating parameters including battery charging , fuelling, ignition, engine rpm, engine oil pressure, coolant temperature,and other funtions.

The Instrument cluster electronics (IKE, EKM) often form an important centralised node on the vehicle computer network connected to many systems whilst holding vehicle specific coding data..
The vehicle instrument panel is fitted with warning lights to alert the driver when a vehicle computer system sees a fault (well, the theory is good !), in reality, some faults may occur in a split second of time faster than the processing speed of the computer controller.
When a vehicle computer sees a fault and logs certain error codes, it may also trigger a dash panel warning lamp to illuminate to notify the operator that a fault has been logged.

Instrument cluster electronics are sensitive to battery voltage. Incorrect reference voltages sometimes caused by a faulty battery or charging system or connection may cause intermittent or strange faults with the dash instrument cluster.

Instrument Panel Warning Light On

Dashboard warning lamps illuminated can have numerous causes warning of impending breakdown.
Vehicle manufacturers DO NOT recommend vehicle operation is continued with a warning lamp or warning message active
- It is a warning for a reason !

Dashboard warning lamps may have different function often with a symbol to indicate meaning and may illuminate Red Orange Amber Yellow Blue Green to signify function.
Vehicle owners manual should specify function

MoT test advisory or fail due to warning light on

Dashboard warning lamp illumination may indicate a fault resulting in MOT test advisory notice or Mot test fail.
(further guidance can be found in the DVSA MoT inspection manuals .

Typical operation sequence of vehicle system instrument panel warning lamp is to illuminate when the key is switched to ignition position as a vehicle system self check is carried out, then should go off when system self check is passed and the vehicle engine is running.

Vehicle Safety Features

Vehicle system control unit software safety features check logged fault codes which may in some cases prevent functionality and cause breakdown :
For example a vehicle self protection feature could stop the engine running when a low fuel pressure fault code has been logged to prevent fuel leakage which may cause fire.

Vehicle Fault Codes

Much can be said about vehicle fault codes this is a link to more information.

A fault code may be logged in vehicle computer contrl unit memory if an error is detected.

Vehicle fault codes often identify that a sensor, actuator, or componenent is not working correctly.

Much of the problem with computerised vehicle diagnostics, fault codes and descriptions found on scan tools during vehicle checks, rests with interpretation of vehicle manufacturers software engineer's cryptic technical terminology (fault codes) in to plain English which can be understood firstly by the technician in vehicle workshops, and secondly, perhaps, more importantly, by the customer who will be paying for the vehicle repair work and is faced with the decision of evaluating costs and benefits of repair versus vehicle replacement.

Open Circuit

A vehicle fault code may log as an open circuit fault if a component is disconnected, unplugged, or there is broken wiring.
The fault code may log as open circuit but will not identify the exact location of a break in wiring etc.

Short circuit

A vehicle fault code may log as a short circuit to ground, or short circuit to power (B+), again this identifies a problem in the circuit but will not divulge location.

Component fault code

A fault code related to operation of a specific component again does not mean that unit is faulty. Fault codes for DPF soot accumulation can be caused by broken wiring to pressure sensors.

Retrieval of vehicle fault codes

Many vehicles require computerised diagnostic equipment with dedicated software to interrogate electronic system control modules and retrieve 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 Mobile Diagnostic Computer (MoDIC), Mobile Diagnostic Interface System (MoDis) or 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. Commercial vehicles, Construction and Agricultural machines may display fault codes on a diagnostic screen but require engineer user code to access.

Read / Delete Vehicle Fault Codes

Fault Code Categories

EOBD ; European Onboard Diagnostic

OBD-11 : American On board diagnostic

Manufacturer Specific

Historical fault codes

Intermittent faults which occur during driving may be logged as a fault code in vehicle computer memory.
A dashboard warning light may illuminate to advise the driver an error has been detected.
Several error codes may be present due to accumulation over time.

Component Disconnection - Open Circuit Fault

Warning: Avoid trial and error random disconnection of vehicle electrical connectors unless you have equipment to reset fault codes.
Trial and error component disconnection may cause fault codes to be logged in control unit memory then requiring diagnostic equipment plug in to clear false fault codes.
Once a vehicle electronic control module has logged certain fault codes this may prevent engine starting or cause erratic running until an adaptive system relearns parameters.

Warning: Disconnection of piezo type diesel fuel injectors whilst the engine is running can be dangerous and introduce further problems such as leaving the injector open causing fuel build up in combustion chamber and potential hydraulic damage.

Vehicle Control Unit Memory Reset

Diagnostic equipment will reset the vehicle computer control system software to turn a warning light off,
however
Warning: If a vehicle fault is still present the check light will often  illuminate again once a driving cycle has been completed unless the fault has been rectified.

No Stored Fault Codes ?

No stored fault codes may result where a vehicle fault or glitch happens too fast for the control unit software to register.
Explained by the instructor of a Bosch engine management training course as : Imagine a photographer taking pictures of a racing car going around a track at high speed, if the shutter speed on the camera is set incorrect or photographer too slow,the car may pass by without any picture being taken.
Vehicle faults may happen fast, particularly sensor faults.
Fault detection may involve use of an oscilloscope to data log the vehicle during operation hoping that the fault will reoccur during testing. Intermittent faults can be both time consuming and expensive to locate.

Vehicle Diagnostic Test - Electronic Health Scan

Vehicle diagnostic equipment is capable of

• Fault code reading
• Actuator testing
Actuator testing can verify wiring circuits by operating the suspected faulty 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.
• Programming and Coding
Some components such as electronic fuel injectors , throttle bodies etc may need adaptation coding to the vehicle ecu.
• Advanced Oscilloscope Electronic System Testing
Electronic signal testing using an Oscilloscope can be carried out when indepth diagnostics are required. Once described by a customer "e; as the difference between a doctor and a surgeon "e;

Vehicle Repair

Following auto electrical or mechanical diagnostic testing, a faulty wiring circuit may be repaired or component adjusted , cleaned or replaced.

Vehicle ECU Memory Reset Procedures

Following repair work the fault code memory of the vehicle computer may need to be reset to clear any stored fault codes.

Check Engine lamp

Check Engine Control Unit (ECU) / Emissions / Engine management warning / Management Information Lamp ( MIL ) / Powertrain Control Module (PCM) lamp illumination indicates that the vehicle engine management  computer control module has identified a fault related to the efficient running of the engine and possibly affecting exhaust gas emissions.
Vehicles with diesel engines may use the glow plug pre heat warning lamp to indicate engine management fault information.
If the check engine warning light illuminates during engine operation then a fault code has almost certainly been logged by the engine control system.

Limp Home Mode

Fault detection may cause the engine warning lamp on with engine control unit strategy to reduce engine power output by reverting to limp home or restricted mode as a safety feature to protect the engine or environment.

Limp home mode is usually be noticed by the driver because of

Poor acceleration

Hesitation or shuddering under acceleration

Restricted performance, eg engine power limited or the engine will not rev above a certain rpm.

Potential ECM warning light on causes

Sensor Inputs and Outputs :

The expected value from each engine sensor is programmed into the software of the engine control module as a parameter on a lookup table referred to as a map. (Fuel map, Ignition map, etc).
An out of range parameter value may represent a faulty component , wiring circuit or mechanical condition.
The check engine lamp may illuminate to notify the driver that a fault has been detected where a value exceeds parameters programmed in to engine controller memory map. Sometimes faults may store without warning light operation.

Actuator Inputs and Outputs

Electromechanical actuators are used to position components.

Engine Sensors, Actuators , and Common Faults

Advanced technology has added lots of control measures to basic combustion engines to increase fuel economy and output power whilst reducing exhaust pollutant emission.

Engine   position sensors :

Cam position sensor

Function: Camshaft position sensor(s) are used to identify cylinder phasing

Crankshaft (CKP)position sensor

Function : Identification of engine crankshaft position and calculation of engine speed and combustion forces.
Frequency and amplitude of signal is proportional to engine rotational speed.

Camshaft and CKP failure Symptoms: Non Start, Misfire

Typical failure : Signal errors can occur due to sensor failure , phonic wheel damage, loose component, wiring connection failure, loss of synchronisation between crankshaft and camshaft(s).

Driver Input Sensors

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

Function :Used to reduce or stabilise idle speed and fuel delivery quantity when engine is off load. Fault symptoms : Worn out switches may result in false (implausible) signals.
May affect truck mounted pto driven mobile equipment operation.

Intake Air Temperature Sensor (IAT)

IAT may be incorporated with air mass meter , with MAP sensor (T-MAP) or seperate.

Manifold Absolute Pressure Sensor (MAP)

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

Temperature Sensors

Engine temperature measurements may be signalled by either negative temperature coefficient (NTC) or positive temperature coefficient (PTC) engine coolant temperature (ECT) sensor or cylinder head temperature (CHT) sensor out of range eg engine overheated.
Temperature signal is processed to compensate fuel injection delivery, egr position and diesel preheat functions to give engine smooth running.
Overheating may cause an engine to sieze.

Coolant warning lamp

Low coolant level. Check for leaks as engine damage may occur due to loss of coolant. A red 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.

Engine Fuel Delivery System

Low and high pressure fuel injection pumps, fuel pressure regulator, fuel injectors.
Fuel systems are prone to faults caused by fuel contamination such as mixing petrol and diesel fuel when refuelling , and general wear.

Diesel Common Rail Fuel Pressure Fault

Modern diesel engines fitted with common rail fuel systems which operate at very high fuel injection pressures may log a fault code when a deviation in actual and desired fuel pressure is present indicating potential fuel leakage.

Turbocharger Boost pressure control :

Turbocharger waste gate position and hence boost pressure control is often actuated by electromechanical solenoids controlling vacuum circuits.
Common faults may include: Worn turbocharger, seized or jammed wastegate,
Turbo boost pressure solenoid loss of vacuum due to vacuum pump wear, split or blocked vacuum pipes, electrical circuit faults with control, solenoid or wiring, mechanical faults.

Lubrication

Oil Pressure warning lamp

Illumination of a red oil pressure warning lamp indicates low engine oil pressure which may lead to major engine mechanical failure ie engine seizure and breakdown.
Possible causes: Low engine oil level, oil pressure switch or circuit fault, Oil pump failure, Oil pressure relief valve fault.

Oil level warning lamp

Certain vehicles are fitted with a red yellow or amber low engine oil warning lamp on the dash and an oil level sensor in the engine sump.

Oil Service Light

Oil service light may come on to advise an oil service is due.
Oil service frequency data calculation may be programmable. Oil service light reset may require manufacturers procedure or diagnostic tester.

Exhaust System

Lambda Sensor

Petrol Engine Lambda Sensor

Lambda sensors detect the oxygen content of exhaust gas and are used as a feedback signal to the engine control computer for control of fuelling.
Lambda sensors can deteriorate due to age or contamination which may cause slow switching resulting in high exhaust CO emissions due to incorrect fuelling leading to driveability problems or Mot emissions test failure.
Lambda sensor fault codes may also relate to problems other than the sensor itself.

Heated Exhaust Gas Oxygen HEGO 4 wire Lambda sensor

The Lambda sensor features a heater circuit. Lambda sensor operation 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 exhaust catalyst.

We supply a range of replacement engine sensors and actuators at competitive prices

Exhaust Gas Recirculation (EGR)

EGR systems function to reduce combustion temperature and oxides of Nitrogen (Note : not Nitrous oxide !).
EGR and Idle speed control ( ISC ) devices are particularly prone to blockage and 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!

Exhaust additive systems & Diesel particulate filters

Exhaust additive systems introduce chemicals in to the exhaust system to reduce diesel soot emissions by passing gases through a Diesel pariculate filter (Dpf). Many of the additives are Urea based and corrosive. Typical faults include damage to wiring from leaking additive.

If failure of an exhaust additive system occurs the likely knock on effect is soot and carbon build up inside the diesel particulate filter.
Dpf blockage may occur over time causing the vehicle software to activate a limp home mode function causing power restriction.

Dpf regeneration is a process where control sofware monitors engine temperature causes the engine to run in a programmed sequence to attempt burn off of excessive carbon and clear the system restriction.

DPF regeneration is not always possible and may require specialist cleaning of the dpf filter or dpf renewal.

Exhaust Brake

Exhaust brakes are fitted to hgv commercials comprising a butterfly flap in the exhaust system.
Flap closes to provide additional engine braking by restricting flow of exhaust gas.

Vehicle Speed Sensor (VSS)

PO720 gearbox output shaft speed sensor fault may cause the speedometer to stop working.

Anti-theft Immobiliser warning lamp

Typically illuminates or flashes red whilst the vehicle immobiliser is active preventing engine start.
OE manufacturer immobiliser systems are often integrated into several vehicle on board computer systems 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, or broken wiring causing CAN signal faults.
In the event of breakdown OE factory immobiliser systems can rarely be bypassed other than by reprogramming.
Aftermarket alarm and immobiliser systems are installed as an added security measure by some owners.
If aftermarket immobiliser systems fail they can normally be removed or bypassed by an autoelectrician to enable vehicle operation.
Warning: You should check with your vehicle 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.

Computerised Diagnostics

Modern Diagnostic equipment with up to date software for modern vehicles is one of a range of tools providing Electronic Health Scan, Fault code reading, Fault code clearing, Actuator tests, ECU interrogation for live data.

Vehicle Diagnostic testing available in Shrewsbury Oswestry Telford Shropshire & Welshpool Mid Wales areas.