Use caution when working on fuel systems, which can remain pressurized for a long period after the key is turned off. No smoking around fuel. Watch for electric fan which can come on at any time. When working on a no-start vehicle, use a neon spark tester to check for spark and ground the coil wire to prevent accidental starting if spark should occur while testing. Watch out for high voltage secondary circuits.
Some people have problems distinguishing the different codes.
There are drawings of the code formats at the end of this section. These are the first things output on a Key On Engine Off test. These are output right after the fast codes. It will show up as a code 10 on most digital testers. The same would happen for a sensor that only went out of range occasionally.