I have a problem with the forward head on my P42. When flushing the bowl will fill but not empty. Aft head works fine. Had the holding tank emptied
recently.

My P42 had a problem with the anti-siphon valve on the aft head. The barb on top of the U fitting was clogged with scale. So even though the solenoid valve was working correctly, air never entered the hose to block the siphon action. Water would slowly run into the bowl and fill the holding tank in a couple of days. If you flush and water immediately enters the bowl, then you probably have this problem. There should be about a three second delay before water arrives and water should continue to flow for a few seconds after you release the button.

The other possibility is that the hose to the holding tank is clogged. Has someone installed a valve and/or check valve in this hose? They would probably be in the compartment with the macerator.

Check the tank vent because if its clogged, itll cause the pumpout to pull a vacuum, preventing more than a gallon or two from being pumped out..AND flushing will pressurize the tank, creating backpressure that prevents bowl contents from going anywhere. So make sure the tank really IS empty.

The most common cause of sluggish or no discharge from electric toilets is low voltage to the toilet...however, the motor would sound sluggish. If yours doesnt I think you can rule this one out.

Sea water calcium carbonate buildup in the head discharge hose can reduce the diameter enough to prevent the the toilet from flushing...and its much bigger problem in warm waters than in cold. Ive seen it reduced from a 1.5 hose to less than 1/2 in only a year or so. It can also clog up the slit in the joker valve in the head discharge.

The discharge impeller may have failed.

If you dont know what or where the joker valve and discharge impellers are, youll find them on the exploded drawing for your toilet. Check the manufacturers website if you dont have it.

You might also make sure that any y-valve in the head discharge line isnt aimed at a closed thru-hull.

The macerator will not create a vacuum in the holding tank because it will draw water and air from the toilet and will empty the tank regardless of a plugged vent hose. Same for the deck pump-out.

Another remote possibility to check for is a problem my boat came with. The hose end that connects to the deck fitting was not properly clamped and had slipped off. This made the deck pump-out completely useless. All it would pump is air because the fitting wasnt connected to anything. If you have a Y connector instead of a Y valve, this problem would disable the macerator too.

Only a very strong pumpout can manage to put enough suction on the system to pull air through the toilet if the vent is blocked...the macerator couldnt do it unless the joker valve in the toilet were so worn that the slit is no longer a slit, but has become a hole. And if any y-valve in the head discharge line is open open to the thru-hull instead of the tank, it would block any air flow through the toilet.

A pumpout pulling air through the toilet is a sure sign that the vent is blocked. That MAY allow a strong pumpout to empty the tank, but it still puts alot of strain on the tank walls. Flushing the toilet against a blocked vent will pressurize the tank, which can result in anything from an eruption in the bowl to blown hose connections to a burst tank.

My understanding of a joker valve is a simple passive rubber check-valve. Passive in that there is no external mechanical device to turn its function on or off. It allows flow from the toilet to the tank, but not back. Why does this device require a strong pump to pull water or air in the normal flow direction?

The standard P42s did not ship with a direct discharge Y-valve. Very environment and swimmer friendly.

Waste from the toilet is just pushed through the slit..waste or water pushing from the other side shouldnt be able to get through it unless its so worn that the slit is no longer a slit, but has become just a hole.

But a pumpout or macerator is PULLING from the other side. Only the top of the joker valve completely fills the hose...its a cup, with space around it. And what happens when you put suction around the outside of the the joker valve cup? The suction doesnt force the slit in the lips open...it pulls em closed.

A really strong pumpout MIGHT be able to pull a little air through the slit if the joker valve hasnt been replaced recently...but not nearly as much as is needed. A macerator isnt nearly powerful enough.

A hissing sound in the toilet during pumpout or using the macerator means two things: the tank vent is blocked...and its probably time to replace the joker valve. The blocked vent should be corrected before using the head or attempting to empty the tank again...the joker valve can be replaced as part of the next annual preventive maintenance.