What causes a DO reading of zero or a negative value?
Check the connector and the RS-485 bus first; only then consider the cap, the sensor itself and the controller port.
| Possible cause | How to check | What to do |
|---|---|---|
| M12 connector not seated | Check that the connector is fully in (listen for the click) | Re-seat the connector |
| RS-485 A/B reversed | Check wire colours (A red / B white) | Swap A and B |
| RS-485 cable broken | Continuity-test the A/B lines with a multimeter | Re-terminate or replace the cable |
| Sensor membrane cap damaged | Inspect the fluorescence cap for cracks | Replace the cap |
| Water inside the sensor | Open the sensor and check for moisture | Replace the sensor (return for repair) |
| Modbus address conflict | Check addresses when several probes share the bus | Assign unique addresses |
| Controller RS-485 port fault | Test with a known-good sensor | Replace the controller board |
What causes a DO reading that is too high?
Bubbles near the probe and a cleaning or calibration problem are more common than a failed sensor.
| Possible cause | How to check | What to do |
|---|---|---|
| Bubbles attaching to the probe | Check whether the probe sits next to an aerator | Move it at least 3 m away from the aerator |
| Auto-cleaning not running | Trigger cleaning manually in the app | See the cleaning fault checks |
| Fluorescence cap aged (>1 year) | Check the replacement record | Replace the cap and recalibrate |
| Calibration offset | Repeat the saturation calibration | Run the calibration flow in the app |
What about a low reading or slow drift?
Work through the installation position and the calibration procedure before replacing hardware. See the sensor installation guide and the RS-485 wiring checks.
A drifting reading is usually an installation or calibration issue rather than a sudden sensor failure.
Related guides
Frequently asked questions
Do I need to register or leave an email to use these guides?
No. All troubleshooting and commissioning guides on this site are open pages with no form and no email gate. The full product datasheets are also open on the downloads page.
Do these pages publish spare-part prices or service fees?
No. These pages publish technical checks, wiring rules and procedures only. For spare parts, service terms or a quotation, contact [email protected].
How far should a DO probe be from an aerator?
At least 3 m. Bubbles attaching to the probe push the reading high; the field handbook gives this as the first check for a high DO value.
What wire colours are used for RS-485 A and B?
A is red and B is white in the SAFEBACK field handbook. Swapping them is one of the first checks for a zero reading.