Bradford White Ultra Low NOx D series gas error code 91 flash
Insufficient combustion air detected
Which models this applies to. This page covers the Bradford White Ultra Low NOx residential D series gas codes.
What is specific to the Ultra Low NOx D series gas platform
Atmospheric gas water heater with a flue damper and a Honeywell WV4472 electronic gas control, which reports faults as a counted LED flash with a three second pause between repeats. The damper codes have no equivalent on units that do not have one, and the codes are not shared with the condensing platforms of other brands.
Your control board does not print the code. It blinks an LED, and you count the flashes against a legend printed on the board or inside the access panel. Count a full cycle before you read it, because the pattern pauses and repeats.
Quick answer
On Bradford White Ultra Low NOx D series gas, 91 flash is a fault. It means insufficient combustion air detected. A first thing to check: Not enough air available to the appliance for the size of its input.
- Applies to
- Bradford White Ultra Low NOx D series gas
- Bradford White Ultra Low NOx residential D series gas codes · Water Heater
- Type of code
- Fault
- Urgency: Stop using it
- First safe check
- Look up the flue for a blockage and confirm the vent length and elbow count are inside the manual limits.
What 91 flash means on this model
The unit has measured that it is not getting enough air to burn cleanly and it says so directly. On an atmospheric unit that is usually the room the appliance is in rather than the flue, and the same shortage is what lets combustion gases collect where they should not.
Symptoms you will notice
- Nine-one flashes, then a three second pause
- Unit will not heat, or heats and stops
- Problem started after the appliance was moved or after building work
- Appliance is in a closet, a small utility room, or an unvented attic
- Any odour of combustion near the unit
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:
- Not enough air available to the appliance for the size of its input
- Appliance installed in a confined space with no permanent opening
- A building envelope tightened up after the unit was installed
- Blocked or obstructed flue
- Excessive vent length adding resistance
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.
- Look up the flue for a blockage and confirm the vent length and elbow count are inside the manual limits.
- Identify how the appliance gets its combustion air. An atmospheric unit in a normal room draws that air from the room.
- Check whether new insulation, sealing, or a new closed door has reduced the air available since the unit went in.
- If there is any smell of combustion, ventilate the space, stop using the unit, and have it looked at.
Do not do these
- ×Do not enlarge an opening or cut a hole to make more air without it being part of a designed and permitted arrangement.
- ×Do not use a small fan in the room as a permanent fix for an appliance that is starved of air.
- ×Do not keep resetting a unit that is reporting this code. It is a combustion fault, not a nuisance fault.
When to call a technician
Stop using the appliance and call today
This one is a safety limit, not a nuisance. The control is shutting the unit down to prevent damage or a hazard, and it will keep doing that until the underlying cause is fixed. Running it anyway risks the expensive failure it was designed to prevent.
- 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.
Checking the flue for a blockage is a homeowner task. Changing venting or the air supply arrangement is a licensed technician job.
What it costs to fix
Parts
Usually a venting correction rather than a part. Flue pipe and fittings $40 to $200.
Labor
Two to four hours to correct a venting or air supply problem, depending on what has to be rerouted.
DIY or technician
Checking the flue for a blockage is a homeowner task. Changing venting or the air supply arrangement 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
- Bradford White Ultra Low NOx residential D series gas codes
- How this model reports the code
- Blinking LED, counted against a legend
- Models in scope
- Bradford White Ultra Low NOx D series gas
- How this platform differs
- Atmospheric gas water heater with a flue damper and a Honeywell WV4472 electronic gas control, which reports faults as a counted LED flash with a three second pause between repeats. The damper codes have no equivalent on units that do not have one, and the codes are not shared with the condensing platforms of other brands.
- 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.