Utica cast iron V-SW (blinking) — Vent Damper End Switch Failed to Make
V-SW (blinking)
Vent damper end switch failed to make. The V-SW light illuminates when the vent damper switch has made; if the damper end switch fails to make in 30 seconds the V-SW light blinks. Verify the damper plug connection/jumper is secure and oriented correctly, the damper motor turns, and the end switch has closed. Carbon monoxide hazard if the burner fires with the damper not fully open. The table classifies it as fault.
If a carbon-monoxide alarm sounds
- Leave the building immediately and get fresh air.
- Call 911. Do not go back inside until emergency services say it is safe.
- Have every fuel-burning appliance inspected before use.
Only after that, call ROMA about the boiler. Official guidance: Health Canada on carbon monoxide.
Applies to
This venting or air-supply fault code is displayed by the HydroStat-style control (LED diagnostics) on MGB-K and MGC Series 4 cast-iron models, as documented in Utica MGB/DXL Boiler Manual, doc 240013750 Rev B (LED diagnostics) — MGC Series 4 in 240013818 Rev A (240013750 Rev B). Codes can differ between controller generations, so always check the manual that matches your unit.
- Utica Boilers MGB-K (MGB Series)
- Utica Boilers MGC Series 4 (MGC-8D/9D)
- Utica MGB-060K
- Utica MGB-095K
- Utica MGB-120K
- Utica MGB-150K
- Utica MGB-175K
- Utica MGB-205K
- Utica MGB-235K
- Utica MGC-8D
- Utica MGC-9D
Controller: HydroStat-style control (LED diagnostics). Source manual: 240013750 Rev B.
Why a code is not a diagnosis
A venting or air-supply code points at the combustion-air path: a blocked terminal, a restricted flue, a failed fan, or a condensate restriction. It does not say which, and running an appliance with a blocked flue is unsafe.
Safe things to observe
- If a carbon-monoxide alarm sounds anywhere in the building, leave immediately and call 911 from outside.
- From a safe distance, check that the vent terminal and combustion-air inlet are not blocked.
- Do not run the appliance while a blockage is suspected.
Do not
- Do not put anything into the vent or air-intake openings.
- Do not remove or modify vent pipe.
How professional service helps
- A technician inspects the full vent and combustion-air path, clears blockages and checks the fan.
- Combustion and safety devices are verified with an analyser and the manufacturer's procedure after any repair.
Where ROMA provides on-site service
On-site service is currently offered in Greater Vancouver, BC, including Vancouver, Burnaby, Richmond, Surrey, Coquitlam, North Vancouver, West Vancouver, New Westminster, Port Coquitlam, Port Moody, Delta, Langley, Maple Ridge, Abbotsford, Chilliwack, White Rock, Pitt Meadows, Tsawwassen, Anmore, Lions Bay, Belcarra, Bowen Island, Mission. Everything else on this site is reference information you can use anywhere in Canada.
Parts and service
Parts commonly involved
These parts appear in the documented text for this alert. A code reports a condition, not a failed part — a technician confirms the cause by testing before anything is replaced.
Burner assembly
The burner assembly is where gas and air meet and burn inside a boiler, and its condition decides whether combustion is clean or makes carbon…
What a technician checks →Flue damper
A flue damper is a motorized blade in the vent connector of an atmospheric boiler that closes when the burner is off to stop heat escaping up the…
What a technician checks →Boiler venting (flue and air intake)
A condensing boiler vents cool, wet flue gases through dedicated plastic pipe that must meet ULC S636 in Canada and the boiler manufacturer's own…
What a technician checks →
Service for this alert
- Gas Boiler Repair
Licensed gas technicians from ROMA Heating & Cooling diagnose and repair residential and light-commercial gas boilers across…
Every boiler part explained — what it does, how it fails and what a technician checks.
Documented for Utica Boilers MGB-K (MGB Series) and Utica Boilers MGC Series 4 (MGC-8D/9D).
Frequently asked questions
Is this code a carbon-monoxide danger?
The code relates to combustion or venting, so treat any CO alarm as an emergency and call 911 from outside. The appliance itself should be checked by a technician before it runs again.
Can I clear this code myself?
Clearing a code without fixing its cause usually just brings it back. Note the code, check the safe items on this page, then call a licensed technician. If your appliance manual describes a user reset for this exact code and it stays away, no call may be needed.
Boiler locked out? Call with the alert text ready.
The brand, model and this alert are passed along when you request service online, so the technician arrives prepared.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.
Sources and verification
- Utica MGB/DXL Boiler Manual, doc 240013750 Rev B (LED diagnostics) — MGC Series 4 in 240013818 Rev A · 240013750 Rev B · accessed September 9, 2026
Facts checked September 9, 2026. Last updated September 9, 2026. Technical reviewer: not yet assigned.