AO Smith 310 series condensing tankless error code 39
Air-fuel ratio rod fault
Which models this applies to. This page covers the AO Smith 310 series condensing tankless gas water heater codes.
What is specific to the 310 series condensing tankless platform
Sealed combustion condensing unit available as direct vent, indoor and outdoor versions. The same two-digit fault numbers apply across the versions, and the fault appears on the control board LED and on the remote controller when one is fitted.
Your control board prints the code as text, for example F01 or EE3, so you read it off the display rather than counting blinks. The printed number is the fault itself, not a position in a table.
Quick answer
On AO Smith 310 series condensing tankless, 39 is a fault. It means air-fuel ratio rod fault. A first thing to check: Soot or contamination on the air-fuel ratio rod.
- Applies to
- AO Smith 310 series condensing tankless
- AO Smith 310 series condensing tankless gas water heater codes · Water Heater
- Type of code
- Fault
- Urgency: Needs attention soon
- First safe check
- Open the burner compartment and look at the rod through the sight glass or inspection port. Note whether it is clean metal or coated dark.
What 39 means on this model
The air-fuel ratio rod is the sensor that tells the control how cleanly the burner is burning, and this code means it is not reporting a usable result. AO Smith lists a sooted rod and a broken wire to it as the two causes.
Symptoms you will notice
- Code appears during a heat cycle
- Unit will not hold set temperature and hunts up and down
- Heating is slow even when the unit is running at full burn
- Code appears soon after a service visit or a vent change
- Flame looks yellow or orange rather than clean blue through the sight glass
Common causes
The code tells you what the control detected, not which part failed. These are the causes we see most often, roughly in order of likelihood:
- Soot or contamination on the air-fuel ratio rod
- Open connection or broken wire to the rod
- Condensate carrying soot and dust back onto the rod
- A failing rod that no longer reports a usable signal
What you can safely check
These are homeowner-level checks. None of them need you to open a live electrical compartment or work on gas.
- Open the burner compartment and look at the rod through the sight glass or inspection port. Note whether it is clean metal or coated dark.
- With power off, follow the wire from the rod back to the board and check the connector is fully seated and free of corrosion.
- Look at the flame through the sight glass. A flame that is not clean blue is the reason the rod is unhappy.
Do not do these
- ×Do not clean the air-fuel ratio rod with a wire brush or any abrasive. It is a sensing element and it has to stay a consistent surface.
- ×Do not replace the rod with a flame rod just because they are physically similar. The two are wired and calibrated differently.
- ×Do not try to defeat the air-fuel ratio control to keep the burner running. It is a combustion safety measurement.
When to call a technician
Book a technician, do not keep resetting it
This fault will not clear on its own. Every power cycle gives the control one more failed attempt, and repeated attempts are what turn a cheap part replacement into a seized or contaminated system.
- This is a fuel-burning appliance, so treat anything involving a burner, gas valve or flue as technician work. If you smell gas at any point, leave the property and call the gas utility from outside rather than continuing to diagnose.
- If the appliance has stopped mid-cycle, is making a noise you have not heard before, or smells burnt, stop reading here and call. Those are symptoms of something the fault code does not describe well enough.
Visual inspection of the flame is a homeowner task. The rod is inside the sealed chamber, so replacement is a licensed technician job.
What it costs to fix
Parts
Air-fuel ratio rod $60 to $160. Board connector and harness $40 to $110.
Labor
One to two hours to replace the rod and clean the burner compartment.
DIY or technician
Visual inspection of the flame is a homeowner task. The rod is inside the sealed chamber, so replacement is a licensed technician job.
Typical US part and labor ranges, 2026. They vary by region, model and contractor, so treat them as a planning figure, not a quote.
Scope of this code
- Code table
- AO Smith 310 series condensing tankless gas water heater codes
- How this model reports the code
- Printed on the control display
- Models in scope
- AO Smith 310 series condensing tankless
- How this platform differs
- Sealed combustion condensing unit available as direct vent, indoor and outdoor versions. The same two-digit fault numbers apply across the versions, and the fault appears on the control board LED and on the remote controller when one is fitted.
- Checked
- Confirm on your own unit
- Find the model number on the data plate, then check this code in the service manual for that model. Manufacturers revise code tables between model years, and a revised board can renumber a fault.