Pentair Pool Heater E05 Error Code: Pressure Switch Open

Quick Answer

Pentair MasterTemp E05 means the pressure switch remained open, signaling water flow through the heater is below the required minimum. The four main causes are low water flow from the pool pump, a partially open bypass valve diverting water away from the heater, a clogged heater inlet screen, or a failed pressure switch. The MasterTemp 400 requires 20 to 25 GPM minimum.

I've serviced probably 200+ MasterTemps and E05 is almost always a flow problem, not a broken part. If you ignore it and keep resetting, you're just stressing your igniter and control board with failed cycles over and over. And if the switch actually fails open when flow is fine, you're running a hot heat exchanger with zero protection. Don't ignore this one.

PentairPoolheaterSeverity: highDifficulty: intermediate82% DIY Success

Time to Fix

30–120 min

Difficulty

intermediate

Parts Cost

$10 – $60

Tools Needed

Phillips #2 screwdriver, Digital multimeter with continuity mode

What Does the E05 Code Mean?

Think of the pressure switch like a gatekeeper that only lets the fire start if there's enough water to keep things cool. The code points to the switch, but I rarely find the switch itself is actually dead. Usually it's just doing its job because a valve's turned the wrong way, the pump speed's too low, or your filter's been overdue for a cleaning for about three months.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Low water flow through heater - pump undersized or underperforming40%

Heater bypass valve partially or fully open24%

Clogged heater inlet strainer screen14%

Failed or stuck pressure switch12%

Dirty pool filter raising backpressure and reducing flow10%

Symptoms You May Notice

E05 flashes on the LED panel within 5 to 10 seconds of pressing the heat button, before the burner ever gets a chance to fire.
The heater lights up and you hear the burner kick on, but it shuts off after 30 to 60 seconds and the display switches to E05.
Pool temperature stays stuck even though the heater appears to be on, because the burner never runs long enough to actually transfer heat.
You reset the heater and it immediately throws E05 again before the first ignition attempt, usually a sign the pump isn't primed or a bypass valve is wide open.
Heater cycles on and off every few minutes on cool nights when the pump's barely keeping up with the minimum flow threshold.

Can you reset a Pentair poolheater to clear the E05 code?

Most MasterTemps clear E05 automatically once they detect proper flow, but you can force a reset by cycling the breaker. Or press the Pool or Spa button to Off on the membrane pad, wait until the fans stop completely, then select your heating mode again. If the pump isn't primed and pushing enough water, the code'll come right back within about 10 seconds.

Tools Required for Diagnosis

Phillips #2 screwdriverDigital multimeter with continuity modePool filter pressure gaugeChannel-lock pliers or pipe wrench for union fittingsFlathead screwdriverReplacement O-ring for heater inlet union

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

Replacement Parts

If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.

Part Name	OEM Number	Estimated Price
Pressure switch42001-0060S · $25–$60	42001-0060S	$25 – $60
Inlet strainer screen42001-0091S · $10–$20	42001-0091S	$10 – $20

Frequently Asked Questions

What does E05 mean on a Pentair pool heater?

E05 is a safety lockout telling you the pressure switch is open. Pentair designs these heaters to be flow-dependent to protect the copper or cupro-nickel heat exchanger. Without enough water moving through to pull the heat away, the metal would melt or crack within seconds. The switch is the only thing standing between a working heater and a total meltdown, so the board stops everything if it doesn't sense pressure. Don't mess with this safety circuit.

How do I fix E05 on a Pentair MasterTemp?

Start by backwashing your filter or cleaning your cartridges. Then check that any bypass valves are sending 100 percent of the flow through the heater. If you've got a variable-speed pump, bump the speed up. If those steps don't clear it, I usually pull the inlet screen and check it for clogs before finally testing the pressure switch with a multimeter. Nine times out of ten it's a flow issue, not a failed part. The switch itself is only about 30 to 40 dollars if you do end up needing one.

What is the minimum flow rate for a Pentair MasterTemp 400?

For the MasterTemp 400, you really want to see at least 40 GPM for the heater to run efficiently, though the switch might technically close as low as 20 GPM. If you're right on the edge, the heater'll cycle on and off repeatedly, which wears out your igniter and control board way faster than normal use. On most VS pumps, setting your heater speed to about 2,800 RPM is the sweet spot I tell all my customers to avoid nuisance E05 codes.

Can a bad pressure switch cause E05 even when flow is fine?

Absolutely. While it's usually a flow problem, these switches live in a harsh chemical environment. High acidity can eat away at the rubber diaphragm inside the switch over a few seasons. If you see water leaking from the plastic body of the switch or the terminals look green and crusty, it's time for a new one. Part number is 42001-0060S. A quick continuity test while the pump's running tells you immediately whether the switch is electrically dead.

Is E05 the same as ERR PS on Pentair heaters?

They're essentially the same fault. E05 is the code you see on the MasterTemp's ignition control board display. ERR PS is what shows up on the automation remote or the newer digital RS-485 panels. Both mean the control board tried to start the burner but the water pressure safety circuit wasn't closed. Diagnosing one is the same as diagnosing the other. Start with your filter and work your way toward the switch.

How long does it take to replace the pressure switch on a MasterTemp?

Honestly? About 20 minutes if you've done it before, maybe 45 the first time. Turn the pump off, cut the power at the breaker, close your isolation valves, then open the heater cabinet. The switch is right there on the water manifold with two wires and a threaded fitting. Unplug the wires, unscrew the old switch, thread the new one in (don't overtighten the plastic body), reconnect the wires, done. Part number 42001-0060S, usually 30 to 40 bucks online. It's genuinely one of the easier heater repairs you can tackle yourself.

Related Pentair Poolheater Error Codes

Code: HUBPentair Pool Heater Error Codes Guide Code: ERR PSPentair Pool Heater ERR PS Error Code: SERVICE-HEATERPentair Service Heater Light Code: RESETHow to Reset a Pentair Pool Heater

Same Fix on Other Brands

Hayward poolheaterIF: Hayward Pool Heater IF Error Code

Models Known to Experience E05 Errors

This repair applies to most Pentair poolheaters with this error code. Common model numbers include:

MasterTemp 400 (Natural Gas), MasterTemp 400 (Propane / LP), MasterTemp 300, MasterTemp 250, MasterTemp 175, MasterTemp 125, Sta-Rite Max-E-Therm 400 (same control board), Sta-Rite Max-E-Therm 333

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 14, 2026