Check whether air can pass through the filter.
There is a sensor located towards the bottom of most condenser dryers and air has to be able to pass through the filter and over this sensor. If the fine pores on the filter become blocked then no air flow passes over the sensor and this can trigger the fault.
To ensure the filer is clear, it can be held under running water and lightly scrubbed clean. If you can gently blow onto the filter and feel the air coming out the other side then this is a good indication that it is clear.