We have a Workmaster 33 bought in 2017 which now has 120 hours. The red exclamation point engine warning light and the regeneration lights are blinking and the tractor won't go above 1800 RPM. I have read that this may be due to failure to regenerate the DPF and/or there may be error codes stored in an ECM. I have warmed it up and tried to force a regen cycle with no luck. It did start one, only to stop about 20 minutes later. I've worked on cars for a long time, but I'm new to tractors. What would be the course of action to get back up and running?