My sub has no power, ther could even be other problems i'm not aware of. so i would like to fix power first

the sub floor must be of the same height, depending on yours ther may be as much as 3/8 difference. the only solution other than leaving it is to remove flooring fill gap and using a floor sander or a belt sander bring the floors even. you will need a straight edge 3-4 ft in length and place from new floor to old and see if there is a gap work until it is gone.. never use particle board for sub floor use
osb or t&g plywood. The floor patch nd leveler is avail at home depot, low's or other type store.

yes it can

The speaker is most likely blown out, and if it is there is no way to fix it, you will just have to replace it

Crackling is usually a sign of a blown speaker or a misaligned coil.

Both can be very difficult to fix themselves.

However you may be able to buy a new speaker .

You would have to dismantle the sub and remove the broken one first.

A goo hi-fi shop or even musical instrument shop may be able to help you with that.

You would need to know the impedance of the blown unit along with its frequency range. That information would usually be in your user manual.

Its not a difficult job but if it is a powered sub then be aware of electrical hazards and ensure it is disconnected from all sources before attempting any work.

it is impossible to make a dual 2 ohm sub only be seen as a 2 ohm load with out damaging the speaker. there is a way but if i tell you how to do it, then "WHEN" not if but definately when you blow up the sub you will be ticked at me and expect me to pay for it so i am bound by law not to disclose it. being mecp certified and an authorized distributor makes me liable for any
mis-information. and the only way to do it is the worst way to do it that may possibly even cause the amp to blow or even worse the sub can and most times does actually catch fire inside the voice coil.

you have to have a dual 4 ohm.sub to get a 2 ohm load at the amp

-H4-21 means:Sub heater lamp abnormally low temperature detection. The setup temperature (about –25°C: Sim 43-1-1) is not reached within the specified time (about 32sec) from turning on the power ON SW. (When the temperature falls below 140°C in the standby mode.)
-Causes:
Sub heater thermistor defect
Sub heater lamp failure
Sub thermostat failure
Control PWB failure
-Check the sub heater lamp blinking with SIM 5-2-2.
When the lamp blinks normally: Check the thermistor and the harness. Check the MCU PWB thermistor input circuit.
When the lamp does not light: Check for disconnection of the heater lamp and thermostat. Check the interlock switch. Check the power PWB and the MCU PWB lamp control circuit.
Clear the display of self-diagnostics with SIM 14.
The H4 errors will not reset by themselves. You must enter service mode to reset this error even if the problem has been repaired. The reset process is outlined below (DO NOT USE ANY OTHER FUNCTIONS IN SERVICE MODE):
Press the following sequence: "#/P" "*" "C" "*"
Type in 1 4
Press start
hit 1 for yes
hit start
Unit should power off and the restart.
If the error code recurrs ther are other problems.

Hello there!
First Things First::

1) RMS quick your sub., if you got extended warranty, for a replacement. ( this is a tipical problem for this spk.) they are aware of this.
2) you can check if something is doing interference on your room , like TV, Microwave near, or other electronic devices.
sometimes this cause extrange problems on sound. SOMETIMES.

is this a new problem or has it always done it?? it mos tlikely needs repairs, but try this first. Hook up ther + of one sub to the speaker + on the amp. Hook the - of that samne sub to the + of the other sub. Then connect the - of the other sub to the speaker - on the amp. This will run the speakers in series and will put alot less strain on your amplifgier. It will not "hit" as hard but it sbetter then having it shut of fall the time. I would also make damn sure that:
Your ground wire is large enough and properly grounded. It should be BOLTED to a solid poiece of metal on your car. Use a screwdriver, whatever so scrape off any rust as the connection should be a clean, BARE METAL connection. Next make sure your power wire and fuse are ok. If you are using round glkass fuses, replace it even if it looks good. I have seen those fuse smelt the solder in the ends. They can look great but really be bad.

SO go through all the basics and if it still gives you problems then it needs repaired. Please dont rate me un-helpful.

I THINK THERE IS A MOISTURE IN YOUR RECEVER KEEP RECEVER IN SUN LIGHT FOR 5 OR 6 HOUR

That's a 4-channel amp, not a 2-channel, which makes me wonder exactly how you have it hooked up to your sub. Is the LED light on the amp coming on? How many subs do you have hooked up to it, and how is it wired?