I have a John Deere z445 mower with new battery and solenoid. It still won't start (just a click). Help?

I have a John Deere z445 mower. It's only 2 years old. It won't start. They replaced the battery and starter solenoid a couple of days ago. It started today, ran for 10 seconds and stalled. After trying starting fluid, it started and ran for 5 seconds and stalled. The battery is good 12.3v, but when I turn the key to start, it stays at 12.3 v and just give and "click" noise (like it's not actually starting). It's in park and the mower deck is not engaged. My gas was from last summer. I've got a mix of old gas, stabil, and new gas in the tank now. Any ideas?