Pentair Pentair MasterTemp 400,000 BTU Gas Pool/Spa Heater FS Error Fix: False Flame Sense

Error code FS on the Pentair Pentair MasterTemp 400,000 BTU Gas Pool/Spa Heater means: False Flame Sense. The flame sensing rod is detecting a flame when no gas should be flowing (the gas valve is commanded closed). This is a potentially dangerous condition indicating either a leaking gas valve (gas flowing past a closed valve and being ignited by residual heat) or a faulty flame sensing circuit on the control board giving a false positive reading. Shut down the heater immediately. If you smell gas, evacuate the area and call the gas company. A leaking gas valve must be replaced by a qualified gas appliance technician. If no gas smell is present, the flame sensing rod or control board circuit may be faulty.

The most successful fix is "Test and Replace Flame Sensing Rod or Control Board" (hard difficulty, ~60 minutes). Estimated DIY cost: $20-$40. HomeMD has 2 community-ranked fixes for this error.

This error is rated critical severity. It is generally not urgent and can often be resolved with a simple DIY fix.

Yes — the top-ranked community fix ("Test and Replace Flame Sensing Rod or Control Board") is rated hard difficulty and takes about 60 minutes. Tools needed: T20 Torx screwdriver, Multimeter (microamp meter), Fine emery cloth, Leak detection solution.

DIY cost: $20-$40. Professional repair typically costs $300-$500. Many homeowners save money by trying the top-ranked community fix first.

The Pentair Pentair MasterTemp 400,000 BTU Gas Pool/Spa Heater has 8 other known error codes, including: AFS (Air Flow Switch Fault), AGS (Auto Gas Shutoff), bd (Board Error), BURNER DEBRIS (Burner Tray Obstruction), BYPASS VALVE (Bypass Valve Adjustment Needed), and 3 more.