If you’ve replaced the bulb and cleaned the quartz sleeve and sensor lens then the only other possible reasons for the continued alarm that I can think of are:
- Defective sensor
- Fault in the controller
- Excess water turbidity
I assume that you’ve eliminated the third possibility. A zero reading from the sensor could be a problem with either the sensor or the controller, but I’d guess that a problem with the former is more likely. Replacing either the sensor or the controller is an easy DIY procedure. If you have a technician come out he/she will only replace the one that has failed, whereas if you do this yourself you’ll be guessing. It comes down to a cost thing, then. If you guess correctly that it’s the sensor, your cost to replace will be less than paying for the service call and the inflated parts cost from the service company.
If you can really replace the unit for $500, vs $300 for the sensor I’d replace the unit, considering that it has been running for 7 years. My quick Google search didn’t see a monitored system like yours available for that low a price though.
If you do replace it, you might consider a version without the sensor, if you’re using the system as a “just in case”. Our tests of our well water never show coliform bacteria, so I’m comfortable having a system without a sensor, although I do check it monthly to ensure that the bulb is working. If you know that there is bacteria in your raw water, then I’d stick with a version with a sensor.