General Motors has delayed delivery of its Canyon/Colorado diesel trucks for an unspecified problem at their “final validation” stage, the automaker told TTAC on Wednesday.

“Those trucks are still in final validations and we hope to ship soon,” said Brian Goebel, a spokesperson for GMC.

More than 60 trucks ordered by customers are in varying stages of production, according to customers who shared order details with TTAC. Several of those trucks’ delivery dates have been pushed back multiple weeks, and many of those orders go as far back as the beginning of August.

According to the automaker, the final validations aren’t related to the emissions certification obtained by the automaker from the U.S. Environmental Protection Agency. Goebel didn’t detail specifically why those trucks didn’t pass GM’s “final validation” standards, but only that the issue was with all diesel models of the trucks and not just specific trims or packages.

Ryan January said he placed his order for a Colorado in August and his production week has been pushed back three times since then. On Wednesday, January said his target production week was re-listed as Jan. 18, 2016, well beyond the initial Fall 2015 shipping date initially quoted by General Motors.

January said GM’s customer assistance wouldn’t specify when his truck would be produced, but that conversations with his dealer revealed the Jan. 18 date.

Meanwhile, Goebel said that GM hasn’t technically passed its initial on-sale date for Fall 2015, although that would require the automaker to deliver those trucks within five days.

Goebel didn’t specify when the trucks would be delivered to customers.

Fleece Performance, a tuner in Brownsburg, Indiana, said they had ordered a Colorado, but that they hadn’t yet received the truck.

Spokespeople for Chevrolet didn’t respond to repeated calls for comment. A spokesman for the Wentzville, Missouri plant where the trucks are built didn’t respond to repeated calls for comment.

“We’re not going to open the gates until we know the truck is ready for consumers,” GMC’s Goebel said.