Various issues with lights seems to be a common topic. My latest deal is weird. Had a taillight burn out so I thought I would replace both as they had not been replaced for years. The store I happened to go to was out of the 3157 bulbs which are called for, but they had several pairs of 4157, which is called a heavy duty replacement. I installed them and then did a test of all lights and a walkaround to confirm. All the signals work normally, front and rear. When I activated the emergency flashers, only the left front bulb flashes, and only the left indicator on the dashboard. I have not yet tried changing back to 3157s to see if that solves the issue, nor have I tried a new flasher. My truck, a 2008 F150 STX, seems to be very sensitive when it comes to bulbs, and putting in anything other than identical to OEM causes odd things to happen. Does anyone else have these kinds of aggravating issues ? I would love to go to all LED lights and add the appropriate flashers if needed, but with the problems I've had so far with non-stock bulbs, I just don't know if it is even worth the effort and expense.

 

Back when I had a 2008 F150, I had sylvania zevo leds all around with an EP27 flasher unit. Never had a problem in many years running that setup.

Im inclined to think there many be some electrical issues with your truck, as that shouldn’t be happening with the bulbs you installed.

 

have you tested/ruled out: voltage/batt/alt and grounds? if yes, i’d try stock bulb part#.

 

The 4157's may have a different resistance or amperage that could be affecting other devices in the flasher circuit. Probably best to start by replacing the 4157's with 3157's.

 

I remember having hyper flash going on occasion with my ‘08 back in the day. Maybe it was the alternator then, because it whined a slight bit and the headlights brightened as well when I goosed the pedal at a stop.

Do check that out. Mine wasn’t an old one by mileage, either. I sold it before it reached 81K miles.