Ok. The p0455 is more than likely due to a faulty/loose/missing gas cap. The p1489 & 1490 are pointing to the low&High speed fan relays, however it is very uncommon that these 2 relays go at the same time, and is more likely a result of a faulty Pulse Width Modulated (PWM) cooling fan relay which controls both Low&High speed relays. This PWM Fan relay has always been a major defect in Jeeps. This relay is located below the right headlamp behind the bumper fascia.
hahahahaha! Headers on a Mustang?! Why bother! Junk it and buy and old GM, they make much better HotRods. If I had a nickle for every Mustang i've beaten with that green Cutlass in the pic, i'd be rich!
Glad I could. Have a great day!