Oasis Tankless Water Heater Won't Stay On After Sitting - Control Board Troubleshooting Guide

Bottom Line Up Front

Your overnight hot water loss with the need to flip the AC heater switch each morning strongly suggests issues with the unit's flow sensor, ignition system, or control electronics rather than mechanical pump problems. The fact that toggling the switch restores function for exactly 24 hours indicates the control system is failing to maintain proper startup sequence overnight, and the electrical reset helps reinitialize the system. While it could be a control board issue, the circulation pump is the most likely culprit and should be your first replacement before exploring more expensive alternatives.

Root Causes

Common causes include a failing flow sensor that's not detecting water demand properly, ignition system problems (for gas models), or heating element issues (for electric models). When your Oasis unit sits overnight, the flow sensor may be collecting debris or the ignition system may be losing its ability to start reliably. The electrical reset from toggling the AC heater switch provides just enough jolt to get the pump moving again.

A secondary possibility is a failing control board that's not sending consistent power signals to the pump overnight. However, both mechanical pump failure and electronic control issues can manifest as 24-hour cycle patterns, so proper electrical testing is needed to determine the root cause. The pump may also be cavitating due to air in the lines, but this typically doesn't resolve with simple switch cycling.

Temperature-related expansion and contraction could also be affecting worn pump seals or bearings, causing the unit to lose circulation overnight when temperatures drop, then function normally once warmed up during the day.

Before You Start

SAFETY WARNING: Always shut off both electrical power and water supply to the unit before beginning any repairs. Turn off all power sources to the water heater - this may include both 120V AC breakers and 12V DC fuses depending on your system and close the water supply valves to prevent electrical shock and water damage.

First, verify which specific Oasis model you have, as pump locations and part numbers vary significantly between tankless and tank models. You'll need to access the unit's service panel, so ensure you have adequate lighting and the unit is completely powered down before inspection.

Check your owner's manual for the correct part for your model (verify with dealer) - this will be a circulation pump with voltage specifications varying by model - check your manual or manufacturer specs for the correct pump type. Common replacement pumps run between $80-150 depending on your model. You'll also want to have basic plumbing tools available: adjustable wrenches, pipe thread sealant, and towels for inevitable water spillage.

Before assuming pump failure, perform a simple test: next time you experience the morning hot water loss, listen carefully near the water heater when you first try to get hot water. If you don't hear the pump running, but it starts after toggling the switch, this confirms pump issues rather than other system problems.

Document the exact switch position that triggers the restoration - knowing whether it's the main power, AC heating element, or circulation pump switch helps narrow down the electrical pathway involved.

The Repair Process

Start by checking the unit's flow sensor and gas valve connections (for gas models) or heating elements (for electric models). Take photos of all connections before disconnecting anything. Check for loose electrical connections to the control board and verify proper water flow through the unit.

Before replacing components, test the flow sensor and control circuits with a multimeter if you have one. With power OFF, check for continuity in the control circuits. For voltage testing, ensure all connections are secure and dry, then carefully power the unit while maintaining safe distance from water. Use proper electrical safety procedures and consider professional help for high-voltage testing. Check that proper voltage reaches the heating elements or gas valve controls. If voltage is present but the unit isn't heating, the heating elements or gas valve may have failed.

To replace the pump, shut off water supply and power to the unit. Disconnect the water lines (expect some water spillage) and remove the electrical connections. Install the new pump in reverse order, ensuring proper orientation - the arrow on the pump housing should point in the direction of water flow toward the hot water outlet.

Apply pipe thread sealant to threaded connections and torque to manufacturer specifications. Over-tightening can crack the pump housing. Reconnect electrical wires matching the original polarity, restore water and power, then test operation. The new pump should run quietly and provide immediate hot water circulation.

Professional Help Needed

Contact a qualified RV technician if voltage testing reveals no power to the pump location, as this indicates control board or wiring harness problems requiring diagnostic equipment. Control board replacement typically runs $200-400 plus labor, significantly more than a simple pump replacement.

Also seek professional help if water line connections are corroded or if you discover multiple pumps in your system - some Oasis models have both circulation and pressure pumps that work together. Misdiagnosing which pump has failed can lead to unnecessary parts replacement.

If your unit is still under warranty, pump replacement should be covered as this represents a manufacturing defect rather than normal wear. Contact your dealer before attempting DIY repairs to avoid voiding warranty coverage on a relatively expensive system.