Hi, I have a 2013 GMC 2500HD, with 197K miles. In 2016 dealer replaced reducdant heater (P21DD) at 86,500 miles. I understand from reading this forum that this is common failure for 2013's. My question is, check engine light came on and code is P21DD, is the failure of the reducdant heater happening again common? Or might this be caused by another issue? This truck is serviced on a regular basis. Have also just replaced the def system thermometer and had filter cleaned. Thank you, Mrs S