I am trying to stop power feedback on a 12volt car lighting circuit, actually the 2 indicators that feedback from one side of flasher unit. My question is what DIODE do I need for this, I have 1N4001's but I think the 1A rating won't work for long as 1 indicator bulb is 21watts which is 1.75A, so 2 would = 3.5A is this correct? Thanks!
Sounds correct, provided that the two blubs flash at the same time. If they alternate (and you trust the circuit that they always will) you will get by with a lower rated diode, maybe even a 1N4001 would survive.
So find yourself a bigger diode from your favourite supplier. I would opt for a large margin, maybe a 5A diode. Be aware that diodes for such currents can have a significant voltage drop, which might cause your flasher to flash somewhat dimmer.
You can limit the voltage drop by using a Schottky diode; the SB560 is rated at 5A and drops only 670mV, so even at 3.5A dissipation will be less than 2.5W.