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Depends upon how well it was taken care of and how bad it was abused. I don't think you can put a definite date or hour rating on anything like this. Onan has a suggested time / maintenance schedule but I have seen few people and companies that actually do it on time. The quality of service combined with the quality of the products used also play a big role in machine life. Use of high quality oil, filters and fuels leads to less problems just as much as routine servicing does. I have a Bobcat I got with 902 hours on it. It was a one owner machine and was serviced regularly and with quality products so I had no fear of buying it. Likewise, I have seen them sold at auctions with under 1000 hours and were not worth scrap price. Onan engines are not cheap to fix or replace if they take a crap big time. The only way to really know what you are getting second or third hand is to rip it down and have a look see at it. If you know where it came from and know how well it was serviced and used, this gives you an upper hand. I've seen these machines with well over 2500 hours on them that were taken care of and still running with no problems. My Trailblazer has close to 2000 hours on the P220 (20 Hp Onan) without a hint of trouble. Of course, I change the oil, Valvoline, and filter, Baldwin, every 30 - 40 hours of run time. I also keep my machines spotless clean which means a lot on air cooled equipment. Best bet is to get it opened up before you put any money down. Check the engine for internal damage such as scored cylinders, pitted piston tops, cam, crank, ect for wear or signs of damage. Also check the generator brushes, commutator, swicth contacts, internal wiring, ect. Burned or overheated wires are usually a sign of a bad machine or one that has been beaten to death or near it. A quick check of a machine is to just pull the covers off the generator side and have look at the wires and switchs, some allow you to inspect the brushes like this as well. If that section looks good and the engine starts easy and runs smooth, use a volt meter to check the outputs. On the welding side, you should have between 60 and 90 volts on an open circuit (will vary by machine and current settings on the CC side. On the CV side, if equipped, voltage should hold to dial settings of the range selected.) 120 / 240 volt AC outlets should have the corresponding proper voltage in the high idle mode. If you have a frequency reading meter, check for 59 to 61 Hz (cycles per second) 60 Hz +/- 1 is OK but it should be right on. Try welding with it if possible. Loose or erratic arc is a sign of trouble. I'd stay clear of machines with over 2000 hours or ones with an unkown history or multiple owners. These are usually puss cases better left alone. Not to say that you can't buy a brand new machine and spin a bearing, break a crank or have a generator fry but it's not too common with Miller. Like anything, it's hit and miss sometimes but if the machine has a good past and looks good, chances are it'll be fine.
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