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I have an Airens Zoom Max 60 with a 25 Hp Kohler engine. Yesterday, it started fine, but cut out after 15 minutes or so of use. No strange noises. It started right back up and ran for a couple of minutes before cutting out again. I don't remember how many times this happened, but after a couple/few more, it required choking before it would start. It eventually (maybe 6 or so total starts) failed to start all together. When I turned the key, nothing happened: no clicks, buzzes, nothing, much like a safety switch were open. The battery is fine and I started it today and it ran for a few minutes on medium throttle, then cut out and wouldn't restart (again, no clicks, buzzes, etc.). I checked the air filter, which wasn't in bad shape. It's possible that it needs a new fuel filter, which I haven't yet replaced, but I don't know that a bad filter would explain the mower not starting.

I would greatly appreciate any ideas on how to diagnose the problem.
Thanks!!

There should be a purple wire on the starter motor
Apply 12 V to it
If the engine cranks normally then you have a switch problem with the mower
From what you wrote it sounds like a bad ground connection or bad fuse / fuse holder .
A clogged fuel filter can make the engine stop after mowing for a while if the mower is using fuel slightly faster than the filter will pass it & be hard to start with out choke.
Foating debris in the fuel tank will do the same thing
When the engine stops, does it surge as if you are running out of fuel , as would happen above or stop dead like you turned the engine off ?

Thanks for your reply. I will try to find a replacement fuel filter to rule that out and will conduct the 12 V test as soon as I'm able.

The engine does not surge at all when it stalls out.

Sorry for the silly question: I assume I put +12 V on the purple wire. Where on the moder do I connect the negative side of the voltage source?

Other way round.
The purple wire should show 0 volts to the engine in every position of the key apart from start when it should go 12 V .
There should be a thick battery cable attached to the solenoid on the starter so you get a bit of wire touch it to the terminal that the purple wire was on and to the battery cable
When this happens the wire in your hands should energise the solenoid which then connects the starter so the engine should crank ifthe starter is good and the battery is good and the connections from the battery to the mower are good .
When you do that & post the results we will provide further instructions so we can work out what is causing your problems & you can fix it with the least cost & effort .

I've done some additional testing and now think I may have been facing at least two problems simultaneously.

First, I replaced the fuel filter. The old one looked like it'd been around a while. To start the mower for additional testing (described a bit below), I used the choke for the initial startup and only once after it died out. I then ran smoothly for ~20 minutes before I turned it off.

Second, in tracking down the purple wire, I discovered that the run relay would click when I messed with the wires to the ignition switch, while the switch was in the off position. I pulled the harness off the switch and found the connector to be damaged, particularly connection S1:

This connector and S1 on the switch look like they suffered from some high current:

I found I could hold the connector in such a way that the mower would start right up (when connected up to the ignition switch). In fact, I got it to start and let it run for ~20 minutes with no problems. This suggests the connector has a short somewhere. I tested continuity on the ignition switch, and it worked properly for off, run, and start (p. 26 in https://apache.ariens.com/manuals/04983500A.pdf).

When I tried to start the mower yesterday, it seemed sluggish. I tested the battery, which is only a few months old, and found it at 11.7 V. So I charged it in preparation for today's testing (described above). I wonder if the problem with ignition-switch connector may have lead to the battery running down?

Do you agree with me that the main problem I'm having appears to be related to the wiring harness connector to the ignition switch?

Replacing the entire harness is an expensive and time-consuming task. Any suggestions for a work-around? My current plan is to cut out the connector and directly connect the wires to the switch, carefully insulating each with heat-shrink tubing and electrical tape. I'd splice on additional wire if needed.

But this leaves me with the concern about what caused the over-current. Any ideas of where I should look for the source of the problem? Or other problems this source may have caused?

Thanks again!

heat like that can be a result of either a good connection with high current, or a bad connection with normal current. Any corrosion causes resistance, and current thru resistance causes heat as a by product. It may be that the circuit passes the current it was designed for when it is clean and normal, but once corrosion sets in it may generate enough heat to self destruct even at the designed load.

Judging buy the green traces evident on the plug your high current load is caused by corrosion at the terminal .
This is why I keep advising to use jumpers to bypass switches with jumpers and using a test lamp rather than a meter .
I just got a craftsman in that would not start , nothing when the key was turned .
Pulled the plug off the key switch to jump the red & whit wires to start the testing and the motor spun
Gave the socket a quick wash with wd40 then a blow dry then slipped it back on and problem solved
Every one is obsessed with numbers but the numbers are by and large meaningless if you do not understand the circuits
Right down to people who post things like 12.87 V as if the .87 means anything other than their DVM is set to read Volts to 2 decimal places .

Your plug is filthy & needs a clean as does the back of the switch
Then you need to have a very close look at the actual terminal with a strong magnifier or just bite the bullet & replace it
Once it is clean / replaced you get contact grease, commonly called di-electric grease and you apply it to the wires and force it through the plug till it oozes out the front around every terminal and refit it on the cleaned key switch .

Thank you for your feedback. I'll get the connections cleaned and apply some contact grease.