Description
Suburban 233665 Water Flow Sensor – ST-42 & ST-60
The Suburban 233665 Water Flow Sensor is the OEM replacement flow sensor used on Suburban ST-42 and ST-60 tankless RV water heaters.
The sensor detects when hot water begins flowing through the water heater and sends that information to the control system. Once sufficient water flow is detected, the heater can begin its ignition and heating sequence.
Suburban’s current factory replacement-parts manual identifies 233665 specifically as the water flow sensor for the ST-42 and ST-60.
Models It Fits
The Suburban 233665 fits:
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ST-42
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ST-60
It is used on both current ST-series tankless water heaters regardless of the difference in burner capacity between the two models.
Does NOT Fit the IW60 / Nautilus
The 233665 should not be listed for the older Suburban IW60 or IW60RL Nautilus water heaters.
Those models use a different flow sensor:
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ST-42 / ST-60 → 233665
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IW60 / IW60RL Nautilus → 233237
Suburban parts listings clearly separate these two flow sensors by water-heater family.
What the 233665 Water Flow Sensor Does
The ST-42 and ST-60 are tankless water heaters, so they do not maintain a tank of preheated water.
Instead, the water heater needs to know when a fixture has been opened and hot water is being requested.
The 233665 detects water movement through the heater and provides the flow-demand signal used by the electronic controls.
When sufficient flow is detected:
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The control system recognizes a hot-water demand.
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The blower and ignition sequence can begin.
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The burner fires.
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The water heater continues operating while water flow remains present.
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When flow stops, the heater shuts the burner down.
The ST-42/ST-60 control center even displays a water-flow icon whenever the unit detects water movement.
Minimum Water Flow
Suburban specifies a minimum operating flow rate of 0.7 GPM for both the ST-42 and ST-60.
If actual water flow through the heater is below that threshold, the heater may not initiate or maintain burner operation even if the flow sensor itself is functioning correctly.
Signs & Symptoms of a Failed Water Flow Sensor
A faulty 233665 may cause:
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Water heater does not recognize hot-water demand
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Burner does not initiate when a faucet is opened
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Water-flow indicator does not appear on the control center
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Heater works intermittently depending on water flow
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Burner shuts off unexpectedly while water is still running
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Heater will not ignite even though propane and 12V power are available
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Water heater operates normally after other components are tested, but no flow signal is reaching the controls
Before replacing the sensor, make sure the RV actually has adequate water flow through the heater.
Problems That Can Mimic a Failed Flow Sensor
A no-flow condition does not automatically mean the 233665 has failed.
Also check for:
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Low campground water pressure
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Weak RV water pump
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Partially closed valves
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Clogged faucet aerators
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Restricted shower heads
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Plumbing restrictions
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Frozen or kinked water lines
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Scale or debris inside the water heater
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Flow below the ST-series minimum of approximately 0.7 GPM
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Wiring or connector problems between the flow sensor and control board
ST-42 and ST-60 Compatibility
The same 233665 flow sensor is used on both ST-series models:
Suburban ST-42
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42,000 BTU maximum input
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Minimum flow rate: 0.7 GPM
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Uses water flow sensor 233665
Suburban ST-60
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60,000 BTU maximum input
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Minimum flow rate: 0.7 GPM
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Uses water flow sensor 233665
Suburban lists 233665 as the common flow sensor for both models in its factory parts documentation.
Related ST-42 / ST-60 Parts
Other common ST-series service parts include:
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521338 – ST-42 Control Board
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521339 – ST-60 Control Board
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161357 – Gas Valve
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233658 – Airflow Sensor
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233676 – Manual Limit Switch
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233675 – Anti-Freeze Limit Switch
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233685 – Temperature Probe
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233678 – Control Center
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233665 – Water Flow Sensor
These components are listed together by Suburban for the ST42/60 platform.
Estimated Install Time
Approximately 20–40 minutes, depending on access to the water heater and plumbing connections.
Tools Required
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Basic hand tools
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Screwdriver
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Appropriate wrench for water connections
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Towels or drain pan
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Multimeter for electrical diagnosis
Installation Notes
Turn off the water supply, LP gas, and 12V electrical power before servicing the water heater.
Relieve water pressure before removing the flow sensor.
When installing the replacement:
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Verify correct water-flow direction.
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Make sure plumbing connections are properly seated.
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Reconnect the electrical connector securely.
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Restore the water supply and inspect for leaks.
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Restore power.
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Open a hot-water fixture and verify that the water-flow indicator appears and the heater begins its normal ignition sequence.
Product Specifications
Manufacturer: Suburban / Airxcel
Part Number: 233665
Part Type: Water Flow Sensor / Water Flow Switch
Fits: ST-42 and ST-60
Water Heater Type: Tankless / On-Demand
Minimum Heater Flow Rate: 0.7 GPM
Does Not Fit: IW60 / IW60RL Nautilus
IW60 / IW60RL Flow Sensor: 233237
Important Fitment Note
The Suburban 233665 is specifically for the ST-42 and ST-60 tankless water heaters.
Do not order this sensor for an IW60 or IW60RL Nautilus simply because those are also Suburban tankless water heaters. The IW60 family uses the 233237 water flow sensor instead.
