Suburban ST-42 Water Heater Control Board 521338

The Suburban 521338 Control Board is the OEM replacement module board for the Suburban ST-42 on-demand RV water heater. This electronic control board manages the operating and ignition sequence of the tankless water heater and is a critical component when the ST-42 fails to start, ignite, or operate correctly.

The 521338 replaces the older Suburban 233725 control board and comes with the outer protective case for installation.

Models It Fits

The Suburban 521338 is specifically designed for:

• Suburban ST-42

• ST-42 on-demand / tankless RV water heaters

This board is not the control board for the ST-60. The ST-60 uses Suburban 521339 instead.

Replaces Part Number

• 233725

Suburban identifies 521338 as the replacement control board for the original 233725 used in ST-42 water heaters.

Signs & Symptoms of a Failed Control Board

A failed ST-42 control board may cause symptoms such as:

• Water heater will not start

• Burner will not ignite

• Intermittent ignition or operation

• Water heater begins an ignition sequence but fails to complete it

• Gas valve or ignition components do not receive the proper command

• Heater shuts down unexpectedly

• Other components test correctly but the water heater still does not operate

Because several different components can produce similar symptoms, check the water supply, propane supply, 12V power, wiring connections, sensors, ignition components, and grounds before replacing the control board.

Estimated Install Time

Approximately 20–45 minutes, depending on access to the water heater and existing wiring.

Tools Required

• Phillips screwdriver

• Nut driver or socket set

• Multimeter

• Basic electrical hand tools

Potential Issues

Before replacing the control board, disconnect electrical power and LP gas to the water heater.

Pay close attention to the existing wiring connections before removing the old board. Photographing the wiring before disassembly can help ensure each connector is installed correctly on the replacement board.

If a new control board does not solve the problem, additional diagnosis may be required. Flow sensors, temperature sensors, ignition components, propane supply problems, poor grounds, low 12V voltage, and wiring faults can all cause ST-42 operating problems that may initially resemble a failed board.

Product Specifications

Manufacturer: Suburban / Airxcel
Part Number: 521338
Replaces: 233725
Part Type: Water Heater Control Module Board
Application: Suburban ST-42 On-Demand Water Heater
Includes: Control Board and Outer Protective Case

Important: The 521338 is specifically for the ST-42. If you have a Suburban ST-60, use Suburban control board 521339 instead.