What does it mean when a honda civic has a flashing cel and random misfire code of po300?

SOURCE: 1995 Honda Civic Spark Plug Wiring

The spark plugs are numbered starting on the drivers side (1) to the passenger side (4), the distributor rotates clockwise, and the firing order is 1 - 3 - 4 - 2

SOURCE: How do you clean the EGR valve on a 99' Honda Civic?

Pull vacuum on egr valve with engine idling and see if engine stumbles and tries to die until vacuum is released. If this DOES NOT happen the egr exhaust passage is plugged will need to remove egr valve assembly and remove carbon from passage into cylinder head.

SOURCE: 2007 Honda Fit with 50K. Intermittently misfire.

Buy some Seafoam from autozone $10 pour half can into half tank drive a little hard on it for a good 10 t0 15 miles. this should do the trick. let me know if it helped!
thanks.

SOURCE: po300 random mulitple misfire detected p1399

If only one of your cylinders was misfiring, you probably would be getting a more specific code than P1399.
Also check the distributor rotor along with the cap and coils and wires (as mrgreenz suggests); the EGR valve to make sure it doesn't leak; your fuel pressure which can cause random misfires if the pressure is too low; and injectors if they are really dirty. If you have really high mileage, I probably would first just change out the plug wires with a new set, then reset the DTC and see if the code comes back.
Hope this is helpful as well.

Testimonial: "I changed the plugs and put an additive to clean the fuel injectors and changed the egr valve. Still running rough when at idle"

SOURCE: i have a code flashing on my ecu for a honda

This is from the first or second google search for "honda civic ecu codes". I'm actually going through this myself.
EF Ecu Trouble Codes
Reading EF ecu Trouble codes
Reading them's pretty simple. When there is a long flash this indicates tens and a short flash indicates ones.

Look under the Passenger seat (under the passenger kick panel for flashing LED - engine code. Short the plug under there.

For example: Flaaaaaash Flaaaaaash Flash = code 21
Flaaaaaash Flash = code 11
Flash Flash = code 2

1) Primary oxygen sensor- Defective circuit or unplugged/ Defective sensor
2) Oxygen sensor "B" - Defective circiut or unplugged/ Defective sensor
3) MAP (Manifold Absolute Pressure) Sensor - Defective circuit or unplugged/ Sensor
4) CKP (Crankshaft Position Sensor) - Defective circuit or unplugged/ Defective sensor
5) MAP (Manifold Absolute Pressure) sensor -Mechanical problem/ Disconnected piping
6) ECT ( Engine Coolant Temperature) Sensor- Defective circuit or unplugged/ Sensor
7) TP (Throttle Position) Sensor -Defective circuit or unplugged/ Defective sensor
TDC (Top Dead Centre) Sensor -Defective circuit or unplugged/ Defective sensor
9) CYP (Cylinder) - Defective circuit or unplugged/ Defective sensor
10) IAT (Intake Air Temperature)sensor- Defective circuit or unplugged/Defective sensor
12) EGR (Exhaust Gas Recirculation) -Defective circuit or unplugged/ Defective sensor
13) Atmospheric Pressure Sensor - Defective circuit or unplugged/ Defective sensor
14) IAC (Intake Air Control) Valve- Defective circuit or unplugged / Defective sensor
15) Ignition Output Signal - Missing or defective ignition output signal
16) Fuel Injector System - Defective circuit or unplugged/ Defective fuel injector
17) VSS (Vehicle Speed Sensor) - Defective circuit or unplugged/ Defective sensor
19) Automatic transmission lockup control Solenoid Valve- Defective circuit or unplugged/ Defective solenoid. A/T Only.
20) Electronic Load Detector - Defective circuit or unplugged/ Defective sensor
21) VTEC Solenoid Valve- Defective circuit or unplugged/ Defective solenoid
22) VTEC Oil Pressure Switch - Defective circuit or unplugged/ Defective switch
23) Knock Sensor - Defective circuit or unplugged / Defective sensor
30) Automatic Transmission Signal- Defective circuit or unplugged/ Defective sensor
31) Automatic Transmission Signal "B"- Defective circuit or unplugged/ Defective sensor
41) Primary Oxygen Sensor Heater- Circiut malfunction
43) Fuel Supply System - Defective or malfunctioning Fuel Supply Sytem
45) System too lean or too rich - Malfunction in the fuel monitoring systems
48) LAF (Lean Air Fuel) sensor - Defective circuit or unplugged / Defective sensor
54) CKF (Crankshaft Speed Fluctuation) sensor- Defective circuit or unplugged / Defective sensor
61) Primary Oxygen sensor circuit- High/ Low Voltage, Or slow response
63) Secondary Oxygen Sensor circuit- High/ Low Voltage, Or slow response
67) Catalyst System Efficiency Below Threshold- Malfunctioning catalyst system
70) Automatic Transmission - Malfunction with Automatic Transmission Controls
71-74) Random misfire in Cylinders 1-4- Condition present Creating Misfire
80) EGR (Exhaust Gas Recirculation) Insufficient Flow Detected
86) ECT (Engine Coolant Temperature) Circuir Range / Performance Problem
90) Evaporative Emissions Contrlo System - Leak Detected In Fuel Tank Area
91) Fuel Tank Pressure Sensor- Low Input
92) Evaporative Emission Control System Control System- Insufficient Purge Flow

Ask your autoparts store how much the parts are and decide if it has a point to even do it, its a 19 year old car. You should also ask if that employee works on the side and would do it for you, if you aren't comfortable.