Once it decides that it needs a regen - that's it. That's why you have to refit the DPF and original software and do a regen (or take the ECU apart and mess with the EEPROM).
Decision to regen is based on few factors: differential sensor and estimated DPF clogging (roughly based on miles driven and speeds and other factors) - only one of those things need to happen for it to decide to regen.
Because your DPF is not there, it was failing to do that (you do not get a message when regen happens), now it estimates clogging to be very bad and message popped up due to that.
Last edited by jacekowski; 03-03-15 at 12:44.