Pentair MasterTemp ERR AGS: Automatic Gas Shutoff

What ERR AGS Actually Means

ERR AGS is the MasterTemp's last-resort thermal protection. While ERR HLS monitors the first pass at 135 degrees F and auto-clears, AGS monitors the second pass at 140 degrees F and performs a hard lockout. When AGS trips, the gas valve closes immediately, the front panel controls are completely disabled, and the only way to restore operation is to turn off the breaker and turn it back on.

This hard lockout is intentional. If water reached 140 degrees F after two passes through the heat exchanger, something is seriously wrong with flow. The design prevents the operator from simply pressing a button and firing the heater again, because doing so without fixing the root cause could crack the heat exchanger or create a scalding hazard.

In practice, AGS almost always follows an HLS event that was either ignored or happened too quickly for intervention. If you see AGS, treat it as a critical flow and sizing investigation, not a nuisance reset.

Step-by-Step Troubleshooting

Owner-Level Checks

Do not immediately reset and retry

1. After AGS trips, the heater panel is dead. Before cycling the breaker, investigate why the heater overheated.
2. Check pump operation. Is the pump running? Is it primed?
3. Check filter pressure. Is the filter severely dirty?
4. Check all valves between pump, filter, heater, and returns.

Identify what changed

1. Did a pump timer turn the pump off while the heater was still firing?
2. Did someone close a valve or start a backwash while heat was being called?
3. Was the heater thermostat set much higher than normal?

Tech-Level Checks

Confirm flow rate after restoring power

1. Fix the suspected flow issue first. Then cycle the breaker to restore the front panel.
2. Run the pump at full speed and verify flow through the heater before calling for heat again.
3. Use a flow meter if available. Verify flow exceeds the minimum for the model: 20 GPM (175/200), 25 GPM (250), 30 GPM (300), 40 GPM (400).

Inspect the bypass valve and heat exchanger

1. A stuck or incorrectly adjusted bypass valve can cause repeated thermal trips. Remove and inspect the bypass assembly.
2. Check the heat exchanger headers for scale buildup. Internal scale reduces heat transfer and causes localized overheating.
3. On older units, check for erosion or deposits in the heat exchanger tubes.

Verify automation and pump interlocks

1. On IntelliCenter or other automation systems, verify the pump is configured to run at adequate speed before and during heater calls.
2. Check that the heater relay or communication address (RS-485 addresses 1-16) is correctly assigned so the automation properly sequences pump-before-heater.
3. Verify fireman's switch wiring is correct if present. The fireman's switch should keep the pump running for a cooldown period after the heater shuts off.

Test the temperature sensors

1. The MasterTemp uses thermistors to measure water temperature. If a thermistor reads incorrectly low, the heater may over-fire before the limit trips.
2. Compare the displayed water temperature to an independent thermometer reading.
3. Test the thermistor resistance against the manufacturer's temperature-resistance chart.

Common Parts That Fix This Problem

• Bypass valve assembly
• Water temperature thermistor
• High limit switch (if it failed to trip before AGS)
• Control board (if AGS trips with normal flow and correct temperature readings)
• Heat exchanger (if internally scaled or damaged beyond flushing)

Model-Specific Notes

• The MasterTemp 400 is most susceptible to AGS trips because its 400,000 BTU output can overheat the exchanger very quickly if flow drops below 40 GPM.
• On dual-heater installations (two MasterTemps plumbed in series or parallel), verify that flow is balanced between units. An unbalanced system can starve one heater while the other runs fine.
• The 12-pin plug connection carries both 240V (red) and 120V (black). After a power cycle, verify all connections are secure before attempting to fire.
• ASME-rated models have the same thermal protection as standard models. The ASME designation relates to vessel certification, not safety controls.

How to Prevent ERR AGS From Coming Back

• Never operate the heater without confirming adequate pump flow first.
• Configure automation to always run the pump at heater-call speed before energizing the heater relay.
• Install a fireman's switch or pump-off delay to keep water circulating after the heater shuts off, preventing residual heat from tripping limits.
• Service the bypass valve and clean the heat exchanger annually.
• If AGS has tripped more than once, do not keep resetting. Investigate thoroughly or call a heater specialist.