Computer controlled engine and emission systems can cause a wide range of problems can cause a wide range of problems. The computer may control the fuel injection system, EGR valve, evaporative control system, and other components. Any of these parts affect the operation of the total computer system and can increase emissions.

Testing computer controlled emission systems

Since computer systems are so complex, special system analysers are commonly used to pinpoint specific problems. Fig. 32-13 shows one type of analyser for checking the operation of a computerised fuel injection system. Following manufacturer operating instructions, the analyser is plugged into the wiring to the electronic control unit (computer). Then, the analyser can be used to determine the cause for most system troubles.

Many computerised emission control systems have a built-in diagnostic system, in’ as much as they will flash a code, when required, which pinpoints faults. A service manual will outline test procedures.

For example, one computer system is checked by grounding a special’ ‘test terminal”. This will cause the computer to flash a special Morse style code from an indicating light. The workshop manual will tell how to read the code (on-off flashes). This provides a quick and easy way of locating problems in a very complex computer system.

WARNING! Do not connect a VOM (volt-ohm- milliammeter) to a computer system unless told to do so by a service manual. There are several components, including the computer that can be damaged by incorrect testing procedures.

Replacing parts in computerised emission system

Depending upon what problem the system tester or self-test mode indicated, you would need to replace faulty components. High emission levels could be due to a faulty oxygen sensor, coolant temperature sensor, throttle position sensor, vehicle speed sensor, intake manifold pressure sensor, fuel injector solenoid, a wiring problem, or by the computer itself.

Before replacing any computer system part, make sure all electrical connections are clean and tight. See Fig. 32-14. A corroded connection could be affecting system operation.

Fig. 32-15 shows a wiring diagram for a modern computer controlled emission control system. Study the location and relationship between the parts. This type diagram is useful when tracing or troubleshooting circuit problems.


After studying your exhaust analyser readings, you must find the source of any indicated problems. Generally, start out by inspecting all engine vacuum hoses and wires, Fig. ’32-16. A leaking vacuum hose or disconnected wire could upset the operation of the engine and emission control systems.

A section of vacuum hose can be used as a stethoscope (listening device). As in Fig. 32-17 place one end of the hose next to your ear. Move the other end of the hose around the engine compartment, along vacuum hoses and connections. When the hose nears a vacuum leak, you will be able to hear a loud HISSING SOUND. Also, inspect the air cleaner for clogging. Check that the air pump belt is properly adjusted. Try to locate any visual and obvious problems. If nothing is found during your inspection, each system should be checked and tested.


Most later model vehicles are fitted with an emission control information decal (label) which is affixed somewhere under the engine bonnet one is shown m Fig. 32-18. The decal will provide important details of basic tuning settings such as idle speed, ignition timing, spark plug gap and distributor dwell. It may even show mixture strength expressed as a carbon monoxide percentage.

Vehicle manufacturers have designed their emission systems so that when these basic settings are correct the vehicle should continue to comply with the emission regulations (Australian Design Rules).

Some vehicles also include an emission component and vacuum hose diagram, which clearly indicates the vacuum hose routings. See Fig. 32-19.

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