Flashing CEL - P0300 & P0421 - 2001 2.0 ES

[dandeeth@hotmail.com]

Looking for a little advice or ideas.

I have a bone stock 2001 ES 2.0 and about two weeks ago I got a flashing CEL. Ran the code and got back a P0300 for a random misfire and P0421 for low cat threshold.

After trying the things below I still get a flashing CEL about once every other day, so if you have any suggestions please let me know.

Things done thus far:

Cleaned EGR Valve
Replaced both front and rear (up and down) O2 sensors
Changed plugs
Bench test coils and wires, which came back fine
Inspected the Cat, looks to be fine

[millyowner]

Some possibilities from the service manual:

* Crankshaft position sensor malfunction
* Spark plug malfunction
* Coil malfunction
* MAF sensor contamination
* Vacuum leak between MAF sensor and throttle body
* Fuel pump malfunction
* Fuel line restriction
* Fuel filter restriction
* Fuel system leakage
* Purge control solenoid valve malfunction
* PCV valve malfunction
* EGR valve malfunction
* Vacuum hose damage or incorrect hose routing
* Related connector or terminal malfunction
* Related wiring harness malfunction