Pool Heater E05 / E06 Error Code — High Limit or Pressure Switch
E05 and E06 on Hayward and Pentair pool heaters are safety shutoff codes, not catastrophic failures. E05 typically indicates a high-limit switch has tripped (the water temperature inside the heater exceeded safe limits), while E06 points to a pressure or flow switch fault (insufficient water flow through the heat exchanger). Both codes can be triggered by real conditions OR by failed sensors — knowing which is critical before ordering parts.
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Common Symptoms
- Heater display shows E05 or E06 and will not restart
- Heater runs briefly then shuts down with error code
- E05 appears on very hot days or after heater runs for extended periods
- E06 appears when filter pressure rises (dirty filter) or water flow changes
- Manual reset clears the code but it returns after 5–15 minutes
- Water temperature at heater outlet is unusually high before shutdown
Most Likely Causes
- 1
E05 — High Limit Switch Tripped Due to Low Flow
The most common trigger for E05 is not a failed switch — it's genuinely high water temperature inside the heat exchanger caused by restricted flow. Dirty filter, closed valve, air-locked suction line, or a slow pool pump means water isn't moving fast enough to carry heat away from the exchanger. The high limit switch trips at 135°F as designed.
- 2
E05 — Faulty High Limit Switch
After correctly triggering several times (due to flow issues), high limit switches can fail in the open position — tripping even when temperatures are normal. Test by measuring continuity across the switch terminals at ambient temperature — it should show continuity when cold.
- 3
E06 — Dirty Pool Filter Reducing Flow
The pressure/flow switch that triggers E06 requires a minimum flow rate (typically 25–40 GPM depending on heater BTU rating). A dirty cartridge or DE filter creates back pressure that reduces flow below this threshold. A filter pressure gauge reading more than 10 PSI above its clean baseline almost always explains E06.
- 4
E06 — Failed Pressure or Flow Switch
The water pressure switch (a small differential pressure device mounted on the heater's water header) can fail open over time due to corrosion or mineral deposits. Test by bypassing the switch temporarily (jumper the two terminals) — if the heater runs normally with the switch jumpered, it's the switch.
- 5
E06 — Air in the Heat Exchanger
Air trapped in the pool heater's heat exchanger dramatically reduces effective water flow, triggering a pressure switch fault. This often occurs after pool winterization/startup, or after any work on the plumbing that introduced air.
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Quick DIY Checks
Do not permanently bypass the high limit switch — it's the last line of defense against heat exchanger damage and a fire hazard. Only bypass it temporarily during diagnosis, then restore it.
Turn off the heater and allow the water to cool before removing any pressure switches or fittings. Water inside a pool heater heat exchanger can be above 130°F.
- 1For E06: Check the filter pressure gauge. If it reads more than 8–10 PSI above normal clean baseline, backwash the sand/DE filter or clean the cartridge before any other diagnosis — this is the most common E06 cause.
- 2Confirm all pool valves are fully open. A partially closed valve on the return or suction side can restrict flow enough to trigger both E05 and E06 without being obvious.
- 3For E05: Let the heater cool for 20 minutes, then manually reset it. Check that the pool pump is running at full speed before the heater tries to restart. Measure supply water temperature at the heater inlet — if it's above 104°F, the heater is heating an already-hot pool and the high limit is responding correctly.
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Try Pro — $7.99/mo- 4Locate the high limit switch on the heater's heat exchanger header (typically one or two push-button manual-reset buttons or auto-reset disc thermostats). Press any manual reset button firmly. Test continuity across the switch terminals — open continuity when cold = failed switch.
- 5Locate the pressure or flow switch (small device with two water ports or a pressure tap, usually on the inlet/outlet headers). With the heater off and pump running, use a multimeter to confirm the switch closes (continuity) at normal operating flow. An open switch at normal flow = failed switch.
- 6Bleed air from the heater: with the pump running and heater off, loosen the manual air vent on the heater's water header (if equipped) until water flows steadily with no air bubbles. This purges air pockets that trigger pressure switch faults.
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Repair vs Replace
E05 and E06 are rarely catastrophic. The underlying causes (dirty filter, flow restriction, failed safety switch) are all repairable. Even replacing both the pressure switch and high limit switch costs under $100 in parts.
Est. Repair Cost
$15–$80 (pressure switch or high limit switch)
Est. Replacement Cost
$2,000–$4,500 for new heater installed
Recommended Tools & Parts
- Buy on Amazon →
Pentair MasterTemp Pressure Switch
Replacement pressure switch for Pentair MasterTemp and Max-E-Therm heaters. Fixes E06 flow/pressure faults. Part 42001-0063S.
$25–$55
- Buy on Amazon →
Hayward H-Series High Limit Switch
Replacement high limit thermodisc for Hayward H-Series pool heaters. Fixes E05 high temp lockout. Confirm BTU model number matches.
$20–$45
- Buy on Amazon →
Pool Filter Cartridge
Replacement filter cartridge — the most common root cause of E06 errors. A clean filter restores adequate flow through the heater. Size by your filter model.
$30–$80
- Buy on Amazon →
Digital Pool Thermometer
Waterproof digital thermometer for measuring inlet and outlet water temperature at the heater to verify actual vs. setpoint and confirm flow rate indirectly.
$10–$20
Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.
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Frequently Asked Questions
- What water flow rate does a pool heater require to avoid E06?
- Most residential pool heaters require 25–40 GPM minimum flow through the heat exchanger, depending on BTU rating. A 400,000 BTU heater typically needs at least 40 GPM. If your pump is undersized, running on a low speed setting, or has a dirty impeller, you may not reach minimum flow even with a clean filter. Calculate your pump's actual GPM output (from its curve) at your system's head pressure.
- My E05 keeps coming back every afternoon on hot days — is the heater damaged?
- Not necessarily. On very hot days (90°F+), if the pool is already warm (88–92°F), the heater doesn't need to add much heat to reach high limit territory — especially if the pump is running slow. Try reducing your heater setpoint by 4–5°F, ensuring the pump runs at full speed when the heater is active, and if you have a variable-speed pump, don't allow it to drop below 2,400 RPM during heating cycles.
- Can scale buildup inside the heat exchanger cause E05 errors?
- Yes. Calcium scale deposits on the heat exchanger tubes act as insulation, reducing heat transfer efficiency and causing the water immediately around the tubes to overheat even when overall flow is adequate. If your pool water has a history of high calcium hardness (above 400 ppm) and the E05 errors are getting more frequent, a heat exchanger scale cleaning (acid wash via a service company or descaling kit) may resolve the problem.