Diagnostic Tools


Compression tester

a compression tester

A compression tester screws in place of a spark plug and will give you a compression reading either in bar, psi or both. The guage will also indicate if this is a good or bad reading.

To use most effectively:

  • Remove all engine spark plugs. This will allow the engine to turn more quickly and use less battery power.
  • If testing a fuel injected engine remove power from each injector, this will stop fuel flooding the cylinders. If non fuel injection find alternative means of stemming fuel flow.

So what does it mean if you get a low reading. Well a number of things can cause low compression:

Block (Head Gasket) tester

the block tester kit

This is a really nice tool, consisting of a liquid and a large looking pippet. Using it couldn't be simpler:

  • pour some liquid in to the unit.
  • with the engine cool open the header tank
  • start the engine
  • use the squeese ball to draw air from the header tank
  • The liquid will turn yellow if exhaust gas is present in the water, confirming a head gasket failure.


Multi Meter


A real must have tool for electrical diagnostics. With so much electrical components on cars many systems can develop faults. A multi meter provides a variety of functions.

Continuity testing

Continiuity testing is probably the most used function, essentialy connect one probe two one end of a piece of wire and the other probe at the oposite end. If your meter beeps consistently then the wire is good. Corrosion or ware from constant bending can cause wires or switches to break down. This will highlight any physical problems quickly.

Voltage testing

Most meters can either test for AC and DC voltages, i encorage you to stop now and understand the difference before continuing! In many situations it is a very good idea to check first if the required voltage is actualy being deliverd. In automotive situations many problems can occor simply from low battery voltage.

To test a cars battery connect the positive probe to the positive battery terminal and the other to eirth or the negitive terminal, you should observe the following readings:

Resistance Testing

Resistance is measured in Ohms and is used to describe how electricity is restricted as it passes through a circit. Resistance can be introduced intentionaly for some very usefull functions.

Resistance is used all over a modern car. It's found in any number of engine sensors, temprature sensors are a prime example. In this situation a temprature sensor will change it's resisince value as the temprature changes. This information is then used by the ECU to determine fuel delivery and for instuments on the dash.

Resistance can also be a real pain where its not needed, as electrical systems age and are exposed to the elements, breaks, repairs/bodges. When this happens the voltage passing through the circut will be adversly effected. For example, as wireing ages corrosion of the copper will be an issue especialy if exposed to salt, water etc. a visible result could be dim head lights. This is because the resistance is causing the voltage to drop. Isolating the problem with a meter can be done by measuring the resistance of the suspect wire, or measuring the voltage coming out of the end.

Test Light

test light

A simpler tool than the multimeter but can be equaly usefull. Its a light with two wires! it will light up in the presence of a voltage. Don't underestimate how usefull a little tool like this can be.

Some more inventive test lights are now on the market wich can also offer a number of other usefull features like the ability to tell you if your touching earth or positive connections some even detect serial data on a wire.

Fault code scanner

basic fault code reader

OBD Overview

So how do they work? Well mordern cars have an ever increasing number of Electronic Control Modules. Manufactures have slowly been forced to adopt standards such as On Board Diagnostics I & II.

These ECM's control everything from the engine to the airconditioning. When a fault occors in a system the ECM that is in charge of that system will register the fault.


Depending on the system in question this may or may not be alerted to the driver, agian this will depend on how major the fault is. For example if the ABS system detects a fault the ABS light on the dash will come on. If the air conditioning system detects a problem with a heater flap you'll only know when the heater doesn't respond in the way you expected.

It is worth noting before going anyfurther that many OBD systems have none or limeted knowladge of the mechanical state of an engine, so if a valve breaks or headgasket goes, you may well get a code but it will be a symtom rather than the cause. Don't forget to use your brain's diagnostic skills too!

Using OBD tools

To investigate you will also need to locate your diagnostic socket, normaly consealed somwhere arround the dash.

Once you have located it plug in the fault code reader and scan the module you suspect may contain a code. Depending on the complexity of your equipment you may or may not have options at this stage. Ultimatly if you recieve a code you will need to make sense of it then clear it.

As i mainly work with VW Audi cars i use vag.com this is an excelent product by rosstech a trial version is avalible from there website but you will need a special cable to connect your pc to the cars OBD connector.

Vag.com is an expensive product for a home user but fault code readers like the one pictured start at under £20 (see links below), which is excelent news for home mechanics.

OBD II readers do not work on all cars please check your car is compatible before parting with your cash! Also check what functions the readers perform to avoid dissapointment.


VDCS (previously VAG-COM)

For VW Audi group cars vag.com is a great diagnostic system, requiring a USB or Serial OBD2 connector vag-com is a fully featured diagnostic application.


It's easy to use and the rosstech website has a great wikki to help you on your way. Scaning for fault codes is a simple case of selecting the control module and clicking on fault codes. Its then a good idea to copy the codes to notpad, clear them and see if they return. If they do further invstigation will then be required.

There are some limitations to this software, you cannot ajust millage on the odomiter required when replacing an instrument cluster. Key coding is also not possible as this requires a dealer network connection.