CEL for glow plug several days ago traveling to NW US. Put 4 gallons of DEF in truck today at Loves after message on being low. Drove about 150 miles and message about poor quality DEF comes on with 99 miles to start of limp mode (65 MPH). Made it about 60 miles to Chevy dealer in Portland, Oregon. We are from Tennessee. At first they said might be the turbo. Tech drove the truck about 40 miles and reported not the turbo. They changed the glow plug and said the DEF is okay because truck has passed the next "check" point on DEF. They said glow plug may have caused it not to regen. One glow plug changed several months ago. Also changed turbo vane position sensor earlier back home. Sensor was under warranty. Glow plugs covered under regular and "extended warranty" by GM. Hope we weren't just sent on our merry way back home. :think:?????