What caused the main fader to become ineffective? Is this a mechanical problem, or a digital issue?

The "PFL" Stands fo r"Pre Fader Level". Just in case you did not realize. so when you press the "PFL" button , and the red light goes out?, You're saying that the hissing goes away and that you have clarity. Yes? and the volume presents no issues? Also correct? Part of that simply, is with the PFL in the on position. You are engaging the fader for one, which will add to an already boosted signal ( the the fader is turned Up?). also you may have your Clip set incorrectly. to set this you turn PFL off, then at your signal input level Dial (usually at the very top in-line with your fader. _ set the volume on your device, then input clip level on the board \ to where the clip light comes on. not a steady light, but a blinking light so make it steady, then back it off a hair. Okay? this whole time make sure your mains are down. or off at the faders. Once clip is set., PFL is in off position, gradually turn your mains up to level. Now, the PFL is for when you have more than one signal. say from an instrument on another channel. unless you have that? No need to use the PFL. If you do have an instrument in line,. then, before you turn PFL on,make sure that your fader on that channel is turned off. -turn on your PFL, then gradually bring that to level. never messing with the mains level.

I know that is a lot of info. but it is a process. that should eliminate any hissing you may have heard.
Good luck!!
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By "fader" we'll assume you mean the sliders on a mixing console, etc. These potentiometers are rectangular in shape and are usually held together by bendable "tabs" that hold the mechanical mechanism together. If you have pushed the control apart, you will need to try and realign the casing and bend the tabs back into place, but be warned. They may also be destroyed and need replacing.

There is NO way to select channels to "Source"... The selections on this mixer are to send the channels to SOLO, Sub1-2 or Sub 3-4 and/or main L-R. You should have the bottom button for each channel punched down to L-R. NEVER connect ANYTHING other than an external effects unit to the INSERT jacks!!!! These jacks are combined output/input jacks with send and return on the same jack. Special insert cables are used for these. An external drum machine should be connected to a line input for one of the channels USING a TRS balanced cable. Page 28 of your user manual shows the block diagram and signal flow diagram. The trim control is the input gain control. Start with those say at 12 o'clock and advance the fader for the channel of interest...you should get signal at the main outs. MAKE SURE that the lines from the mixer to the amps are BALANCED TRS type cables... You cannot use unbalanced cables for this. The user manual explains balanced cables and what they look like. I just repaired one of these mixers that some foolish people plugged stuff into the MAIN INSERT jacks and blew out the driver that drives the MAIN insert which kills the input to the MAIN output drivers. There is one small chip between the AMAIN fader and the sub next to it and one has to remove every knob and take the unit all apart to get to it... It is a tiny surface mounted part the size of a matchhead with 8 leads... not for the faint of heart to repair. As a test of this part, you can plug a st of headphones into the RIGHT MAIN insert jack and drive audio into a channel... turn trim control up fully for the channel and the fader on full, and set the PAN control to 12 oclock and punch down the L-R button for that channel. You should hear the audio in ONLY the left ear... Now try the phones in the LEFT INSERT jack... you again should hear the audio in the left headphone only (these jacks are not stereo like the headphones). If you cannot hear anything, then likely somebody has blown that driver in your unit... It is painful and time consuming to repair this... there are hundreds of knobs to pull off and put back on... dozens of screws and cables and to get it all back together. Also make sure ALL of your interconnected equipment is powered from a SINGLE receptacle or power module. Read your user manual thoughroughly. You can get one online download if you need it.

only 2 effects can be used at a time. you assign the effects in your lil digital section in the master part of the board by turning the black knobs so the lit up numbers change to the effect you want then push the black button to lock it in. at which 2 of your aux's ( the last 2 i think its 5 and 6 ) control the SEND to your EFFECTS and the fader below the selections control the amount of effects goes into the MIX.

Check the FX to main level to make sure it is not too high. Also check the main FX fader level. Try turning each of these down and see if that corrects the issue. The FX fader should control the volume level of the FX globally (mon sends as well as main outs), and the FX to main should only affect the mains output. Make sure FX to mon1 and FX to mon2 are turned all the way down.
Also check what FX program your using and that the FX IN button is depressed, try starting w/ #10 (small room) and see how that sounds.

You probably wrecked the faders putting De-Oit in them!!! NEVER use that in pot and faders... It is for switch contacts only.
You have ONE chance to save them. From Home Depot get a can of CRC226 in the electrical dept. Put some of that using the spout into each fader near the middle and work the faders. The CRC226 costs about $2.70 a can. Make sure it is the 226 as there are OTHER CRC products there and only 226 is the one that is suitable.

The De-Oxit will have cleaned out the lube of the faders and if you are unlucky, one or more of the wipers will have folded back which is fatal... The CRC226 is an electrical lube that I have restored faders and pots with... It has many other uses including lubing plier joints...

The MON faders ONLY control the output to the MON L and MON R jacks, NOT the split use of MAIN/MON outputs. You MIGHT try varying the MONO fader and see if it varies the MON part of the split output.
Best to use the unit as MAIN L and R to main speakers and use a stereo amp or POWERED monitors driven from the MON L and R jacks. With this, the MON faders will control your monitors as you would like them to.
ALSO make sure ALL equipment connected together is powered from teh same recepacle power system. Powering interconnected equipment from receptacles at opposite ends of the room is asking for damage due to ground bounce and faults.
PLEASE read my tip on here about a HAZARD of speaker polarity if you use Speakon to 1/4 inch cables.

Removing all the knobs from a section of the board where the bad fader is can be a hassle.and sometimes some of the nuts from jack or screws from some connectors.

You are probably better to order the part from Mackie to get the exact part. Most are made by ALPS and I get ones for some equipment from DigiKey.com or Mouser.com. However, it does take a bot o knowledge to get the correct part and sometimes requires cutting the shaft of one that is too long.
One MUST match the mounting holes, resistance value, length of throw AND the "taper". Yours will probably be a standard "Audio taper" for the main output.

The utput fader has probably been broken. If the knob takes a hit, it breaks the resistance element. The part is inexpensive but it takes a lot of work to access and replace the part.

its obvious that the fader in question needs replacing ..parts are available from www.peavey.com