If it is a RO/DI, at a minimum you need a new DI cartridge or resin as yours is way past exhaustion.
As I mentioned before, you need to know the tap water TDs and RO only TDS too since the RO membrane is the workhorse of the system and does 90-98% of the work. If the membrane is bad or not seated properly in the housing it could be making the DI do all the work which is not good.
First, get a clear glass drinking water glass and wash it by hand in extremely hot water so it has no dishwasher soap residue or anti spotting agents on it or hard water spots. Then since we suspect your DI is exhausted get a gallon jug of distilled water and triple rinse both the glass and your TDS meter in distilled water then check the TDS of the distilled water in the glass to make sure it is 0 or very close to it. This will ensure the meter is clean and fairly accurate.
Next draw a sample of your tap water in the glass and insert the meter allowing the reading to stabilize before writing it down. Triple rinse the glass and meter in distilled water then let the RO tun for at least 5-10 minutes and take a sample of the RO only water before the DI filter and again let the reading stabilize before writing it down. You should be able to disconnect the 1/4" line leading from the RO membrane to the DI filter at the DI end to draw this sample if you don't have a DI bypass valve. Again triple rinse the glass and meter in distilled water one more time before drawing a RO/DI sample directly from the system after it has been running several minutes then write down this last reading, triple rinse the meter and glass a final time in the distilled and put the meter away clean and capped and the glass upside down so it stays clean for next time.
What are all three TDS readings? This is the first setp in troubleshooting the RO/DI system. If you want to go further you need to know your water pressure and your water temperature and measure your exact waste ratio to ensure the system is operating efficiently. All of these tests are critical to good DI life.
If it is RO only you still need two readings, tap water TDS and RO only TDS and you still need to do the triple rinses to make sure the meter is clean and accurate. If it does not show 0 TDS +/- in distilled water the meter needs to be calibrated or replaced. If it is an RO only system why might I ask do you have two prefilters unless you are using a granular carbon which I hope is not the case as carbon degrades with use and turns to dust when in granular form and can add TDS and potentially foul the membrane if the last prefilter is not very low micron.
