1999 toyota corola code po300 and missfire under lode at 2000 rpm

PO300 is a random misfire code. Generally, a coil needs to be replaced. The DIM in your car should specify which one. (My 2000 S80 did). Easy to change.

No-- it does not run better

Your letting in unmetered air & the oxygen
sensor sees that & the computer adds fuel
& your rpm goes up for awhile

Are the spark plugs over 3 years old?
Is the fuel pressure at motor correct
Does the misfire set a PO300 code
Does the check light stay lite or blink
Is the egr system working as it should
Any vacuum leaks,exhaust,evap system,
intake.
O2 Sensors over 100,000 miles,front ones
PCV vacuum leaks

See if you can get a road test with a professional
scan tool along & look at cylinder misfire counts &
fuel trim

The p0172 is due to an O2 sensor in the exhaust seeing too much oxygen, the po300 is a missfire present and p0304 is cylinder #4 missfire, check plug and wire to #4 and repair, this should also take care of the p0172, if a cylinder is missing it is just pump air into the exhaust thus causing the code p0172.

for these code you can try:

Remove and reinstall the gas cap, clear the code, and drive for a day and see if the codes come back.

Inspect the EVAP system for cuts/holes in tubes/hoses.

Inspect for damaged or disconnected hoses around the Evap purge solenoid.

Check and/or replace the sensor.

Check and/or replace the purge valve.

Have a professional use a smoke machine to detect leaks.

p0300 should be multiple missfire. check for compression these engines tend to bend valves very easily if ran on old gas or a batch of bad gas. causing it to missfire in more than one cylinder. they also crack heads between valves if overheated.

Just cleaning a sensor will not help it, the first thing is to check for a bad injector, swap injectors with another cylinder and see if the misfire move, but first make sure the fuel preasure is good,

These are misfire codes.This means that your vehicle is missing.The 300 is a random misfire and the 305 is for cylinder 5,and the 306 is for cylinder number 6.Check the plugs if they are good then you could have bad coils.They are top of the plugs.Hope this helps.Good luck.

make cranshaft gear has not spun under timing cover ive seen it happen before

usually a p0300 is a random missfire code due to a fuel problem probaly bad gas , use some in tank fuel treatment such as techron or lucas injector cleaner, after adding fuel disconnect negative terminal to batt, to clear codes and see what happens

Flasher? You mean blinker?