reaching and maintaining operating temperature is crucial for proper DPF regeneration . bad thermostat is common and will result is blocked DPF. 35D doesnt have a thermostat on egr cooler but instead a divertor valve inside with a vacum actuator. make sure divertor valve moves freely. if your motor hasnt been reaching proper temp for a while due to bad thermostat. you probably have a seized divertor valve as well .you can lube it and free it up you have to remove egr cooler to change thermostat anyway.