P0123 Code

My daughter has a 2003 Jeep Wrangler and tells me the check engine light came on and the vehicle started missing. We checked the code and received a p0123. Can anyone tell me what I should check or replace?

Thanks

Scott

You can find the codes explained here: http://myjeeptj.com/2008/04/20/1998-2006-jeep-tj-diagnostic-codes

Holy guacamole an almost 6 year resurrection! That might be a record.