I have a 2005 Accord, 4 cyl, that I recently replaced the serpentine belt on and a couple days later it slipped off. The original belt never slipped off, so I am wondering what could be causing the new one to slip.

The tensioner pully is putting tension on the belt and the wear marks on the pully are lining up correctly within the good range, so it seems like that shouldn't be the problem.

Any other ideas out there that I might want to look at?

 

is the belt installed correctly as in all the belt ribs are on all the pulleys?

 

A pulley that is not angled correctly can cause this... one that perhaps has a collapsed bearing. can you check to see if all pulleys are parallel and aligned in the same plain?

If the belt is to long for your system it can loosen and fall off because it will cause the tensioner to be out of range. correct belt correct and tensioner working correctly?

 

Lost power as I was typing this yesterday.

Make sure

A: The belt length and # of ribs match.
B: All the accessories turn freely.
C: Turn the pulleys by hand; make sure they are square to the accessory with no wobble or damaged and dinged ribs.
D: Make sure nothing is getting in the way of the belt. Wire ties, electrical lines, hoses, etc.
E: You have all the ribs in each pulley; if you have difficulty use wire ties and tie the belt to the pulley until you're ready to put it on the tensioner arm then cut.
F: The tensioner arm and idler pulley spin free and do not need lubed.
G: The tensioner arm puts out the correct tension.
H: The pulleys are clean and do not have crap/oil on them.