Happened to me too. It means the particulate filter needs to be cleaned. Let it run and it will burn it out and you will be good to go.
Watch out though, the exhaust gets pretty hot while doing it. Using high sulfur fuel, long idles, and long periods of disuse will cause this condition. Take it out by itself for 30 minutes at high speed.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW