Pentair Pool Heater ERR AFS Error: Air Flow Switch Fault

Quick Answer

Pentair ERR AFS means the combustion air pressure switch in the MasterTemp did not close during startup. The blower must reach operating speed and generate sufficient negative pressure before the heater is allowed to ignite. A failed combustion blower motor, blocked vent or exhaust flue, or a failed air pressure switch are the three most common causes.

PentairPoolheaterSeverity: highDifficulty: intermediate74% DIY Success

Time to Fix

45–150 min

Difficulty

intermediate

Parts Cost

$20 – $250

Tools Needed

Multimeter, Screwdriver set

Wrong code?

Not seeing ERR AFS on your display?

Select what your appliance is actually doing and we'll take you to the right guide.

combustion air flow fault

HaywardAO

blower overheat fault

HaywardBO

What Does the ERR AFS Code Mean?

ERR AFS is a combustion air flow fault - the blower failed to reach speed or the vent is blocked, so the air pressure switch stayed open.

Most Likely Causes

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

Combustion blower motor failure or seized bearings40%

Blocked exhaust vent or intake louvers24%

Failed combustion air pressure switch14%

Bird nest or debris in exhaust flue12%

Wiring fault to blower or pressure switch10%

Symptoms You May Notice

ERR AFS displayed on panel
Heater blower starts but shuts down before ignition
Heater completely silent on startup - blower not running
Heater worked intermittently before hard ERR AFS lockout

Can you reset a Pentair poolheater to clear the ERR AFS code?

After clearing the vent obstruction or replacing the blower motor or pressure switch, perform a soft reset: press ON/OFF, wait 30 seconds, press ON. Allow the blower to reach full speed before the ignition sequence begins - if ERR AFS returns within the first 30 seconds, the blower is still not reaching adequate pressure.

Tools Required for Diagnosis

MultimeterScrewdriver setFlashlight

Diagnostic Checklist

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

Visually inspect the exhaust flue outlet and the combustion air

Visually inspect the exhaust flue outlet and the combustion air intake on the heater cabinet. Look for bird nests, leaves, rodent debris, or any physical blockage.

Initiate a heat cycle and listen for the combustion blower to

Shut off power and remove the blower motor access panel

With the blower running at speed

Test voltage at the blower motor terminals during startup

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
Combustion blower motor assembly472736Z · $120–$250	472736Z	$120 – $250
Air flow pressure switch42001-0057S · $20–$45	42001-0057S	$20 – $45

Verify part fitment for your Pentair model

Repair vs. Replace Calculator

Appliance Age (Years)7 Yrs

Labor Approach

Our Verdict

REPAIR

Estimated Cost: $135 (Parts) vs $3000 (New Machine)

Frequently Asked Questions

What does ERR AFS mean on a Pentair MasterTemp heater?

ERR AFS stands for Air Flow Switch fault. It means the combustion air pressure switch did not close during startup, which the heater interprets as the combustion blower not generating adequate air flow. The heater cannot safely ignite without confirmed combustion air flow.

Can a bird nest cause ERR AFS on a Pentair pool heater?

Yes. A nest in the exhaust flue or air intake is one of the most common seasonal causes of ERR AFS, especially at the start of the swimming season. Always inspect the flue outlet before performing any electrical or component testing.

How do I know if my combustion blower motor is bad?

Listen for the blower during startup - it should spin up audibly within the first 30 seconds. A silent blower, one that hums without spinning, or one that spins slowly and stops points to motor failure. You can also spin the blower wheel by hand with power off - a seized wheel confirms a failed motor.

How much does a combustion blower motor cost for a Pentair MasterTemp?

Replacement combustion blower motor assemblies for the MasterTemp range from $120 to $250 for the part. Professional installation adds $100 to $200 in labor. The part number varies by BTU rating so confirm your specific model before ordering.

Is ERR AFS a safety hazard?

The ERR AFS lockout is itself a safety feature - the heater will not ignite without confirmed combustion air flow, which prevents carbon monoxide buildup from incomplete combustion. Do not attempt to bypass the air flow switch. A blocked vent is a carbon monoxide risk even if you could override the lockout.

Related Pentair Poolheater Error Codes

Code: HUBPentair Pool Heater Error Codes Guide Code: ERR IGNPentair Pool Heater ERR IGN Error Code: RESETHow to Reset a Pentair Pool Heater

Same Fix on Other Brands

Hayward poolheaterAO: Hayward Pool Heater AO Error Code Hayward poolheaterBO: Hayward Pool Heater BO Error Code

Models Known to Experience ERR AFS Errors

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

MasterTemp 400, MasterTemp 250, MasterTemp 300, MasterTemp 125

Last verified for technical accuracy on March 14, 2026