If the E90 error appears on the display, test the water with test strips. The reason for the E90 error in the hot tub may be due to high levels of calcium (or other minerals) in the water. Calcium can very quickly contaminate the filter cartridge, the filter pump and other equipment in the motor base of the hot tub. If the error persists and you have already checked for all possible causes and replaced the cartridge, we recommend a simple test: 1) Unscrew the cartridge, including its housing, from the wall inlet. 2) Turn on the filter and try to feel the water flow from the inlet with your hand. If the water is flowing, the circulation pump is working and the problem may be poor water quality (e.g. high calcium content) or a dirty cartridge. If the water is not flowing, the fault is probably with the circulation pump itself and will need to be repaired. If you are interested in a paid out-of-warranty repair, please contact us using our online form.