Some problems will cause the MIL to illuminate after only one test fails. Other emissions tests must fail twice consecutively before the MIL comes on. If the problem is intermittent, then the light may never come on, but a code will still be stored in the vehicle's computer. Non-emissions problems can also set codes, but they do not turn on the MIL warning indicator. Emissions related OBD2 codes can be read with a handheld computer called a scanner or scan tool.
Automotive technicians use a scanner extensively when diagnosing modern computerised cars.
Some automobile manufacturers such as Chrysler and General Motors allow almost all the computer data to be read by any scanner. Each part of the five-digit code tells something about the problem. The second digit is either the number 0 or 1. The number 1 indicates the code is specific to that manufacturer. The third digit may indicate a system such as the ignition system, fuel system, or transmission control system, while the last two digits represent a specific code.