My 99 es 300 ran absolutely perfect, then today it started idillling rough, the check engine light began flashing on and off, Checked codes and getting a P0303 misfire, and a P0171 system too lean bank one error. Are they related ? does anyone have any advice on where I should begin ?

 

Your #3 cylinder (Rear, middle) is not squirting enough fuel, causing you to run so lean, you're mis-firing, or you've got lame spark & you're mis-firing.
Pull the intake off & check that the fuel injector wiring, and the fuel injector itself.
$$$ is on the fuel injector wiring.
BTW, run to an autoparts store with $10-15 & buy some NGK 5553's. Just say, "I need six, NGK five five five three's. No, five FIVE FIVE three. You got it!".

And um... Clean the IAC valve, EGR valve & throttle plate as basic mantinance while the intake is off, Clean the inside of the upper/lower intake as far as you can reach & gain 5-10bhp from it. (Well not *gain*, but gain back some stock power & responce)

 

UPDATE.

Changed out the plugs and still running rough, switched the coil pack from cylnder 3 to cyl. 4 and sure enough got a P0304 code instead of a P0303 code.

Culprit >. Bad coil pack, installed new one now car runs like new again.

 

93 es300
Do you guys know what my HVAC codes mean? 11,12,13,14,21,22,41,& 43??