In short, the Diesel Exhaust Fluid (DEF) reductant heater failed which requires the entire DEF tank assembly (part 85138746) to be replaced on my 2020 GMC Sierra 3500. This cause the truck to go into "limp" mode where it will not travel above 65 mph, and after another period, the speed continues to be reduced down to 25 mph. Prior to the speed being reduced, there is a warning message that appears stating "Service DEF System See Owner's Manual" and a warning light illuminates in the gauge cluster. Any severe reduction in speed that can cause the truck to go "limp" is a safety concern. In addition to being a safety concern, some quick internet research reveals that this has been a known problem with GM's DEF system since at least 2019. The electrical connections are too small for the current which supplied the DEF tank heater, causing the system to short and trigger the truck into limp mode which renders it useless. The replacement part is on national backorder, and it has been on backorder for years. My truck is completely useless until this part (or a better-engineered replacement) arrives at the dealer, and there are hundreds if not thousands of people waiting on this same, defective part to get their trucks back on the road. People are losing their livelihood and the means by which they support their families. GM has known about this defective part for years and has done nothing- neither engineered a better part nor issued a recall with some kind of workaround. They haven't even been able to stock the defective part, and this is not just a supply chain issue as new trucks are still being built. This problem was not inspected by any police or insurance agency as it did not cause an accident or injury in this case.

- Pittsburgh, PA, USA

While towing a fifth wheel camper, the exhaust was so hot it melted the camper directly behind it. Was caught in side mirror before anything caught on fire.

- Sioux Falls, SD, USA