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I have a yth18542, that's about 3 years old, and it will try to turn over, but the flywheel never spins. If you rock the mower back and fourth, sometimes it the flywheel will advance one rotation, but then it stops again. I've already replaced the battery, and the new one is at 12.7v, so it's not a power source issue. Anyone had this issue?

Remove the spark plug, ground the plug wire with a jumper lead, and try turning the engine over. Report back what happens. When you report back, be sure to include ALL engine numbers.

I have the same problem. Briggs and Stratton engine family LBSXS.5402VL in a 2020 YTH18542 Husqvarna rider. Pulled the spark plug
and the starter turned the engine easily. Replaced the plug and the engine only turns to resistance. Wondering if I have a broken valve?
Can turn the engine by hand but there is strong resistance at one position. Model #33R777 Type 0030 G1 Code 200303ZD

Coug, check your valve clearance.

Figured it out. The camshaft has a mechanism that opens a pressure relief valve. The mechanism breaks, pretty frequently from what my local shop told me, and the camshaft has to be replaced. Mine is still under the Husqvarna 3 year bumper to bumper warranty, but Briggs wants the motor shipped to them, at my expense, to inspect it. My mechanic said he's had several of these motors come in, with the exact same problem, and Briggs has yet to cover any of them. I'm arguing with Husqvarna about it now, but don't see it going anywhere.

Bse1simmons said:

I have a yth18542, that's about 3 years old, and it will try to turn over, but the flywheel never spins. If you rock the mower back and fourth, sometimes it the flywheel will advance one rotation, but then it stops again. I've already replaced the battery, and the new one is at 12.7v, so it's not a power source issue. Anyone had this issue?

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Take it she's a rider.

Pull the oil stick and smell for fuel.

Cougwatch said:

Can turn the engine by hand but there is strong resistance at one position.

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As suggested check valve clearance. Also pull oil stick and sniff for fuel smell.

BEfore you go spending money
When was the valve lash checked / adjusted ?
Too big a gap will cause the same symptoms .
So you could end up spending $ 200 to find it is your fault .
Lack of proper service is an owners problem & not a warranty item
Checking the valve lash is an important part of the first service which from what I have seen is rarely done .
At least 20 times a year some one comes in with your problem having thrown better than $ 400 worth of parts at it .
Because cars ow all have bucket adjusters or hydraulic lifters , to a modern person the idea of valve lash is totally foreign .

Rivets said:

Coug, check your valve clearance.

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Valve clearance was OK but did find the compression relief mechanism on the cam shaft to be broken.