CAR CLINIC | How to use vehicle diagnostic codes to your advantage

The idea that a motor car can tell its owner what's wrong with it is intriguing. Cars have been doing this for decades by means of disconcerting noises, steering aberrations, smoke signals and general misbehaviour. But that only worked if the owner had mechanical sympathies, and could understand his car's language.

As cars became more complicated, the average owner's diagnostic skills became inadequate, and even professional mechanics were hard-pressed to keep abreast of technological advances.

Fortunately the advent of electronic control units with their attendant sensors opened the possibility of communicating directly with the “brain” of a vehicle. In fact, by comparing the information supplied by the sensors with the acceptable patterns and values stored in its memory, the computer itself should be able to lead us to the source of the problem. Thus the pipe dream of self-diagnosis became a reality.

Though the concept was developed in both the US and Europe, it was the serious air pollution problems experienced in California which motivated manufacturers to put an on-board diagnostic (OBD) system into production.

Initially aimed at assuring compliance with emission control laws, the system was subsequently expanded and refined, and in 1996 its second generation, known as OBD2, was made compulsory on all new vehicles sold in the US. In 2001 the EU followed suit with its own system, EOBD, and from there it spread to the rest of the world in the increasingly globalised motor industry.

Now we have reached the stage where just about every new motor vehicle sold in our country has an OBD port, often located somewhere around the front passenger footwell (that is, the driver's footwell on the left-hand drive cars of many industrialised countries). It is a sizeable female multiplex connector, usually with a cover of sorts, sometimes plainly visible, sometimes tucked away under the dashboard or behind the glove compartment. It is meant to receive an OBD scan tool which will enable you to retrieve information from the car's computer(s).

The kind of information that can be retrieved has been diversified and expanded over time in step with the growth of digital control functions. Among the most important items of information that can be obtained by means of a scan tool are the diagnostic trouble codes (DTCs) which are logged on the car's computer whenever “limp mode” has been activated by the computer, often accompanied by “check engine” light appearing on the instrument panel.

Limp mode is a safety feature to protect the engine and transmission from further damage in cases when the computer receives perturbing information from a sensor. Symptoms of limp mode, besides a glowing check engine light, include a drastic reduction in engine power, achieved by limiting engine speed and cutting boost pressure, and the transmission being blocked from shifting up beyond a certain gear (on vehicles with automatic transmission).

One should bear in mind, though, that a check engine light can show up for hundreds of different reasons.  Because the check engine light is glowing doesn't necessarily mean the car is in limp mode. We will return to this topic in a future article.

If limp mode was activated, it becomes vitally important to establish the reason. This is where the DTC logged on the computer comes into play. DTCs are standardised, five-character, alphanumeric codes where each field conveys certain information. A full list of codes can be downloaded for free from the internet.

As an example, in the code P0335, the first field is the letter P which tells you it is a powertrain-related fault, the second field is the digit 0, which tells you it's a generic (or global) code, not manufacturer-specific. The third field is the digit 3, signifying  the problem is in the ignition system. The fourth field is the number 35 which indicates a malfunction in the crankshaft position sensor circuit.