null Your SEO optimized title

M7 Spa Heater and Sensor Set Troubleshooting and Replacement

The Balboa M7 system did away with the old-style pressure switches and also the temperature sensor inside the hot tub thermowell.

With the M7 style heater, there are 2 equal sensors that are built right into the heater manifold.  These replaceable sensors take on different functions depending on the direction of water flow.

Get the M7 style sensor here.

Horse Sense
Warning: Working on your own appliances, cars, covered wagons and hot tubs as well as anything else electrical or mechanical carries with it severe risk of injury or property damage. These tips and suggested procedures are free and for informational purposes only. Proceed at your own risk. It is best to use the services of an electrician or tech. Hot Tub Outpost is not liable for any damage or injury that may result.

Pressure Switches - A Thing of the Past

pressure switch pastThe pressure switch is the single item most likely to fail in an electronic contorl system.
Why? Because the micro switch can fail, the calibration thumbwheel can accidentally be turned or go out of alignement over time, recalibration is required for higher altitudes and the rubber water bellows can become brittle and crack.  That little 1/8" water inlet hole can also become clogged. Improper water chemistry can wreak havoc on pressure switches.

If your system has a pressure switch, see our Pressure Switch troubleshooting guide page.

In these pressure switch type hot tub systems, a temperature sensor is typically mounted in the tub. This may lead to unpredictable temperature accuracy. 

These sensors can also be difficult to replace. (If you ever need to replace a temperature sensor in a thermowell mounted to the spa shell, the rear of the sensor will need to be foamed back in after replacement to avoid incorrect readings from the cabinet temperature).

m7 technology

M7 Technology

  • Heater Tube
  • Heater Element
  • Smart Sensor Technology
  • Standardized Sensor Placement
  • M7 Technology for Increased Reliability

priming-mode-balboa.pngM7 Control Sensors

With M7, both sensors are identical. Sensors monitor each other via software to regulat spa funtions including water flow and temperature.  The sensors assure a safe operating environment for the heater element.  Included in this technology are multiple high limit safety systems and functions that always remain active.  This technology also eliminates excessive cycling and automatically adapts to flow direction.  The system can detect flow restrictions like a dirty filter, blocked suction fitting or closed service valve.  M7 can also minimize calcium buildup on the heater element.

These sensors unscrew and come out. They come with a small o-ring and cap. It also has a red and black wire with a small plug that has a tab that needs to be pushed in to release it from the circuit board.

M7 Control System Functions

How M7 electronics and systems work is that the spa first starts up in priming mode and the pumps are manually checked during startup (4 minutes) to be sure they are all full of water and operational. At that point the pumps are all primed and the heater is full of water too. (Priming Mode). The M7 system sensors go into calibration mode on startup. The sensors will look for temperature difference between the 2 sensors.

Then the pump needs to be run for 2 minutes to check water temperature.   If the set temperature is higher than the displayed current water temperature, and the spa is in standard mode, then the control system will call for heat and the heater should start up with the pump on low speed (or the circ pump operating).

During normal operation and in standard mode, the pump may come on every 30 minutes or so to sample the current water temperature.  In economy mode, the pump does not come on to check current water temp every 30 minutes. It will then only come on during the programmed filter cycles.  In Sleep Mode, the water temperature is also not sampled between cycles and the spa only heats to within 20 degrees F of the set temperature and only heats during the programmed filter cycles. (SLP appears on display then)

m7 heater technologySo the water temperature reading of M7 systems will not be current if the spa is in Economy or Sleep mode - so keep it in standard mode.  To read temp, if not already in standard mode, switch to Standard and then turn Jets1 on for at least 2 minutes.

During the heater startup process, fault message will appear if the temperature differnce between the 2 sensors is greater than 2 degrees F. The error codes SnS or Sn may be displaying the sensor error.

During actual heating, a difference of 3 degrees F or below, between the 2 sensors is considered normal.  A difference of 6 degrees F between the sensors indicates a possible flow problem and the heating will shutoff for 1 min. before the startup process is repeated.  After 5 low flow faults in the last day, the heater will be disabled until a button is pressed for reset.

High Limit Protection

There are 2 levels of overheat protection in M7 systems:

110F degrees - water temperature at lowest sensor, all output relays are turned OFF and OHS or OH error appears until reset.  Note this will auto-reset if temperature drops below 107F.

118F - Heater temperature at either sensor - All output relays turn OFF and OHH or HH error appears. Topside panel reset. Press any button at 107F or lower.

Freeze Protection

Note that the system also has automatic freeze protect at 44F degrees.  ICE or IC error may come on and all the pumps and blower if equipped will turn on in an effort to keep the water moving/heating until the temperature reaches 45F degrees for at least 4 minutes. The sensor freeze protection is always functional except for total sensor failure, but the aux freeze protection is always functionsl if so equipped.

Sensor A Failure - SNA or SENSOR A SERVICE RQD

The sensor connected to "A' is not working and the system is shut down. Check for loose plug or damaged wires.  Switch sensors A and B at the board (for techs). If message changes to SnB or SENSOR B SERVICE RQD, then replace sensor, if not the problem could be with the circuit board.  Same for Sensor B error.

SnS Sensor Sync Error

The 2 sensors may be out of balance if SnS appears.  The the pump ON just before heater ON, the sensors differ more than 2F degrees, that means one or both sensors are several degrees off so the sensor set must be tested (see M7 technical service manual).
A verified sensor out of balance occurs when SnS is flashing. 

Dr or Heater May Be Dry error

If one of the sensors has increased 4F degrees or more during the first 90 seconds of startup, then a possible heater dry condition may have occured (air trapped in the heater manifold..) so the system shuts down out of safety. Check water level, flow restrictions, dirty filter, closed valves, air pocket and pump prime to be sure all systems are normal.

DrY is the Heater DRY SERVICE RQD - verified dry heater message

This happens if the Dr error happens 3x in a row without a successful heating cycle so the system is shut down.


This type of error means that during heating there is a 6F degree or greater difference between the sensors that might indicate a low flow condition (for 230v spas) - this is 4F degrees for 120v spas.


This is the verified low flow message if HFL has happened five times in the last day and heating has been disabled.  Check water level, flow restrictions, air bubbles, closed valves and that the pump is primed.  Press any button to reset (there is no auto-reset).


The spa water overheat error message is OHS and it means one of the sensors has detected 110F degrees incoming water.  This can happen if the filter cycle settings are improperly setup so the spa is constantly cycling (did you mix up AM and PM?). Otherwise maybe it is really hot outside?  System will automatically reset if temperature goes below 108F.


This happens if the temperature inside the heater reaches 118F degrees.  The system will shut down for safety reasons.  Press any button to reset once water temperature drops below 108F.

Most Important Points for M7 System Troubleshooting

By the time you receive an error message which requires a manual reset, the erro has been tested and verified many times.  All error messages other than sensor failure are the result of either low water in the heater or low flow through the heater.


hot tub sensors spas Troubleshooting M7 Temperature and Hi Limit Sensors

The topside control error code(s) can help narrow down a sensor issue on an M7 Balboa control system such as VS300, VS500, VS501, VS504, VS511, VS515, VS520, VS521, VS523, VS525 and other M7 packs.

 Learn more about M7 sensor replacement and find the sensor here.