For many years cars have logged any faults that occur in their engine management system. The most advanced systems record not only the fault, but also engine operating conditions at the time the fault occurred.
These systems normally need a dedicated data reader (eg a
manufacturer's own service tool) to read them. But nearly every car has a simpler way of accessing these codes. By linking two pins in the diagnostics plug, most cars can be made to flash their diagnostic codes on the dashboard Check Engine light.
For example, two short flashes followed by four longer flashes
might indicate the fault code number '24', and finding out what '24' means is as simple as looking up a service manual. Even basic manuals have these codes listed.
Cars sold or built in the United States after January 1, 1996,
use what's called an OBDII diagnostics system. 'OBD' stands for Onboard Diagnostics and represents a standard that allows certain engine data to be downloaded, including fault codes.
(Well it's kind of a standard - see the 'OBD - Oh Bloody
Difficult?' breakout box.)
