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the light will flash a 12 code 3 times then tell you the stored codes then a 12 code when the stored codes are read, so if you only see a 12 and the forth time is a 12 then there is nothing wrong with your truck. the 12 code is just telling you its in diagnostic mode.
Well, that's the check engine light I think. It is saying something is not normal. You can read out a trouble code to give you an idea of why it is on. Turn the ignition on, but engine stopped, and with a piece of wire bridge terminals TE1 and E1 in the service terminal box, at one rear corner of the engine compartment.
The check engine light will then blink. A steady blink 4 times per sec is "all normal".
Trouble codes are 2 digits, and these will be blinked - first digit - pause - second digit - repreat.
So the oxygen sensor fail code (a common one) is 21, and will show 2 blinks - pause - 1 blink - repeat.
I would check for fuel,(pressure and injector pulse), spark, and compression, before checking any codes. Lack of any of these basics can result in codes, and then you waste alot of time chasing a symptom, instead of the actual problem.If more people followed basic diagnostics, which always come back to these three basics, rather than chasing codes first,they would find no starts much more quickly.