Complete newbie needs a little advice

If you replace the two resistors in the original circuit then the blinking LED will be much brighter but not blinding. The LED in series with the power to the 555 will brighten and and dim.
The output high voltage of the 555 will be +10.8V and the output high voltage of the circuit to the ECU will be +0.827V.

ok well then that might do it, it can't be over 1v going to the ecu or else it's still going to trip the check engine light