It starts up just fine. The mower blades are not engaged... but when I let up on the clutch/brake, the engine dies - even when it's in neutral.

What is wrong with it?

If you are sitting on the mower at the time, it sounds like the seat safety switch is not working properly.

You can bypass it to tell for sure. But make sure you hook repair it or hook it back up once you figure out the problem. It is an important feature.

My JD Sabre model 2048HV (Hydrostatic Vehicle) (bought used) just started doing the same thing. At first, the starter would not engage, so I tested the brake/clutch safety switch with a tester, and it was bad. So I replaced it, and it started just fine after that. However, when I went to disengage the brake/clutch pedal, it would immediately die. I could tell that the engine was being grounded electrically, by the way it shut off. Further investigation found that the previous owner had bypassed the seat safety switch. This couldn't be the problem, I thought, since it's acting as if you are seated. But on a whim, I undid the wires, and retested the tractor. It now starts and runs just like it should. What changed, I have no idea, but at least it's working now. And to think I was going to tape down the brake/clutch safety switch...