Head will not turn on , tester shows power coming in at 14.5

Probably.
Code 300 is for a random miss, and yes the extra pressure could be from a leaking head gasket.
You may be able to pull the spark plugs and find coolant residue on one or more of them. A shop could check for exhaust gas in the radiator too.

You can check every fuse on the car in 5 minutes, without even pulling the fuse out to inspect it. You just need a cheap test light, one with an electrical lead with a clip and a needle type probe.

On the tops of all fuses (maxi-fuses, regular fuses, & mini-fuses) are two indents with metal showing, one at each end of the top.
For the under hood fuses, clip the tester lead to the negative post of the battery. Touch the positive battery post with the probe to check your ground clip is good-tester should light up. Now turn the key to on, not start, and touch the tester probe to one end of a fuse in the little slot-it should light up-(almost) every fuse on the car will have power going through it with the key in on. Now you probe the other end of fuse: If fuse is good, tester will light on both ends of fuse. A blown fuse will only light the tester on one end of fuse. If both ends light, fuse is good, and move to next fuse.

For the fuses inside the car, just clip the tester lead to a good ground point-a metal screw or metal bracket under the dash. Again with key in on, touch a fuse end in that testing slot. If tester doesn't light,. check or reposition the ground lead till you see the tester light up. Now touch each fuse on both ends. A good fuse will light the tester on both ends of fuse, a blown fuse will only light tester on one end of the fuse. There, you are done. You not only have checked all fuses, you have verified that power is going through the fuse panel.

There may be one or two fuses that won't light the tester on either end with key on. Some cars have an interior lights fuse that only gets power with the head or park light switch turned on. And some cars have a "crank" fuse that only gets power when the key is in crank or start. (A fuse for the radio will certainly have power with key in on.) There are a few fuses that are hot at all times (like brake lights fuse, and most if not all of the maxi-fuses). But with key on, (almost) every fuse should show power and light up a test light. good luck.

It should be charging at 14 to 14.5 volts when the car is running. SInce that's what you're getting, your alternator is good, and the battery is on it's way out.

It can be easily identified only if you can use a continuity tester- ohmeter in a multimeter to check the leads. If the wires are short or open this can be confirmed. Remove the puysh switch and check for power to the terminals to confirrm this doubt. Please take care when mains supply is present

OK, I know you checked the fuses. So, do you have power at the inputs? Check with a voltmeter
or continuity tester. If so, do you have a remote wire connected to the REM lug. A remote wire brings a low amperage signal, usually from the head unit (CD player), back to the amp to tell it to turn on. You can simply jumper from the + side of the power lug over to the REM lug and if it turns on it means
you need to find the POWER ANTENNA output lead from your deck and route it to the REM lug on the amp. The Power Antenna wire is usually Orange on decks and is low amperage so a 14 to 18 gauge
wire will work to run to the amp. Hope this helps. If after all this still no power I'm afraid it bit the dust.

Overheating is problem, not codes. Codes only secondary from overheating.
Borrow cooling system pressure tester.
Replaces cooling system cap.
Pump up above cap pressure.
Watch that gauge does not go down.
If it does, then leak.
If you can't find it, try filling with coolant again, and over pumping.
Check for water in spark plugs from head gasket.
Run engine.
If gauge oscillates, then bad head gasket.
Always replace thermostat after overheating.
Make sure belts tight.
Charge battery.
If when you try to start, if voltage drops below 10, then either battery bad, start bad, or engine binding.
If it goes back to 12 when you stop cranking, it is not the battery.
If the engine is binding, it will not be easy to turn by hand, even with plugs out.

Your breaker is not tripped.
Your light bulb is working.
You have a working on-off switch that has two brass screws and a green ground screw.
That is a single pole switch. Black wires go on either brass-colored screw, and whites are twisted together.
If switch has 3 screws and ground screw, connect black wires to dark-screw and one of the brass screws.

First you should test your wires.
You have 2 cables.
Both cables have black, white, and bare copper ground wires.
One cable goes to light.
Other cable comes from breaker box.

Cable from breaker box has black-hot wire and white-neutral wire.
Turn on power and test black wire to white, and then test black wire to ground.
Tester should light up with both tests.
If tester does not light up, then breaker is suspect, or there is a short in nearby box.
Track back the short towards breaker, going one box at a time, each time testing black to white and black to ground.

Next, inside the switch box, remove the switch and connect both cables without the switch.
Black to black
White to white
Turn on power, does light turn on.
If light turns on, your switch is suspect. Connect new switch. One black wire to each brass screw, and then tie white wires togethr.
If light does not turn on, your light is suspect. Open light fixture and test black to white and black to ground again.

If light tests with power, then check your bulb
If light has no power, then wire between switch box and light is suspect.

HOLD ON!!! The Klein 69127 is an automotive tester to be used for voltages between 6V-24V, AC or DC. The higher voltage in your home could possibly damage the unit as well as yourself. It would be safer for you to use the non contact tester to verify voltage Klien NCVT-1

lots of problems can cause this check rings are not gone by getting a compression tester and putting it in the spark plug hole and turn the engine over if pressure goes over 100 psi 7 bar then the rings are ok if below im afraid rings have had it also check to make sure the head gaskets is not blown by lose of coolant or oil coming out of head good luck

Through my past experience I mwould remove the solder point at the input power B+ line to primary winding FBT. If the HOT has shorted there will be no power.Straight to the point if you know where the HOT is , you can desolder the middle pin or collector of the HOT transistor. If you desolder the pin which result the power led light turn up. That indicates either FBT or HOT has shorted