FLO on a hot tub,
in plain English.

What FLO actually means

FLO — and its cousins FL1 and FLC — means the control board is not seeing enough water moving past the flow or pressure switch. The board responds by shutting the heater down, because running a heater with no water past it is exactly how heater elements die.

So the code is a protection message, not a breakdown. Something is restricting flow, or the switch that measures it has stopped telling the truth. Those are two very different repairs and the difference is most of the diagnosis.

Check these four things first

In that order. Most FLO errors never get as far as a service call.

• The filter. By a distance the most common cause. A filter can look fine and still be flow-dead — pleats pack with body oils and fine grit long before they look dirty. Pull it out completely and run the tub for a minute. If the code clears with no filter in, you have your answer.
• Water level. Below the skimmer intake and the pump starts pulling air. Top it up to the middle of the skimmer opening, not the brim.
• Valves. Slice valves in the equipment bay get closed for a repair and not reopened. Look for a handle that is out of line with the others.
• Air lock. Common after a refill. The pump runs, sounds normal-ish, and moves nothing because it is spinning air. Loosening a pump union slightly to bleed it usually clears it — and if your tub shows PRR or PROG after a refill, that is priming mode trying to do the same job automatically.

When FLO is a real fault

If the filter is out, the water is up, the valves are open, the pump is moving water and the code is still there, the remaining candidates cost money:

• The flow or pressure switch has failed. A cheap part reporting nonsense to a healthy system. This is the good outcome.
• The circulation pump is failing. Turning, drawing current, not moving what it should. Often audible if you know what to listen for.
• A genuine restriction inside the plumbing. Scale, debris or a collapsed line. Less common, more involved.

Chasing that with parts instead of a meter is how people buy a pump when they needed a $40 switch. The $185 service call covers the trip and the diagnosis, and you hear the actual fault and the price before anything is replaced.

Where we stop

If the tub is also tripping its breaker, that is a different problem wearing the same clothes — see a hot tub tripping the breaker . Electrical work on a spa circuit is licensed work in Michigan and Dr. Spa does not do it. We diagnose everything on the spa side of the disconnect and tell you plainly which side the fault is on.

Related codes

HL and HFL and DR and DRY frequently show up alongside a flow problem, because restricted flow is what trips a high limit in the first place. The full error code index covers all twelve.