Pentair Pentair MasterTemp 400,000 BTU Gas Pool/Spa Heater E05 Error Fix: Stack Flue Sensor Over-Temperature

Error code E05 on the Pentair Pentair MasterTemp 400,000 BTU Gas Pool/Spa Heater means: Stack Flue Sensor Over-Temperature. The exhaust stack flue temperature sensor has detected that the exhaust gas temperature is too high. This is typically caused by insufficient water flow through the heat exchanger — the heat from combustion is not being adequately transferred to the water, so it exits through the exhaust at a higher than normal temperature. The most common root cause is a dirty pool filter restricting flow through the heater. Other causes include a partially bypassed heater (bypass valve too open), scale buildup inside the heat exchanger tubes, or a low-fire burner condition. Clean the pool filter. Check the bypass valve setting. If the problem persists, the heat exchanger may need an acid flush to remove internal scale (professional service required).

The most successful fix is "Check Refrigerant Charge and Repair Leak" (hard difficulty, ~90 minutes). This fix costs nothing. 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 ("Check Refrigerant Charge and Repair Leak") is rated hard difficulty and takes about 90 minutes. Tools needed: Electronic refrigerant leak detector, Manifold gauge set, Vacuum pump, Brazing torch.

DIY cost: $0. Professional repair typically costs $300-$600. 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.

Error code E05 also appears on 1 other Pentair model: Pentair UltraTemp Heat Pump Pool Heater. The fix may vary by model, so check each model's specific page for tailored repair steps.