You need 3 TDS numbers to troubleshoot a RO/DI. Tap water TDS, RO only TDS before the DI and final RO/DI TDS, none means anything without the other two. I can only assume the 12-14 is the final RO/DI but the most important number by far is the RO only TDS since the RO membrane does 90-98% of the treatment, if it is bad the DI will not last worth a hoot since it should only be doing 2-10% of the work.
