Controller shows dIF or DELTA ERROR; solar does not engage even on sunny days.

Safety: Turn off power to the controller before moving sensors or changing wiring.

• Check sensor readings: With the controller powered, compare displayed pool and roof temperatures. On a sunny day, the roof should be significantly warmer than the pool.
• Verify sensor locations: Confirm the roof sensor is mounted on or near the Fafco solar collector, not shaded or touching cold surfaces. The pool sensor should be in the plumbing, not exposed to air.
• Check wiring order: Open the controller and ensure the pool sensor is on the POOL terminals and the roof sensor on the ROOF/COLLECTOR terminals. Swap if they were reversed.
• Adjust differential setting: Enter the controller setup and verify the temperature differential (often 4–8°F). If set too high, the controller may rarely call for solar.
• Replace faulty sensor: If one sensor reading is clearly wrong, replace it with a Fafco-compatible sensor and retest.