Zanussi Upright Freezer not frost free odd problem

I've got a Zanussi upright freezer not frost free . An engineer came out through the insurance and pulled the freezer out to get a serial number from the back with the freezer still running.

He replaced the door seal later that week.

I found out the next day that the red defrost warning light was on and the door seal was not fitted correct and the motor had been running all night to try and keep it freezing which did not work. I fixed the seal and the freezer cooled back down to -18 no problem.

A few days later the red defrost warning light was back on. Checked the door seal all was fine and I could here the motor running but the temperature was going up not down. Turned the freezer power off for 15 min and back on and this fixed the problem.

A few days later the same thing happened again and the motor was still running but the temperature was going up and not down. I turned the freezer power off for an hour this time and it fixed the problem for a longer period time.

Last night the red warning defrost light was back on again. Motor was running but not freezing at all. I turned the freezer off for an hour and back on but this time it took the freezer all night to come back down to -18.

What could be causing this?

Thanks

Has it got a fan in the base that circulates the cold air? These sometimes seize up with dust & fluff.

Thanks for the reply. No it doesn't have a fan on this model.