Bosch A4: Flue-Gas / Safety Fault
What the Bosch A4 error code means, what causes it, and why it's a job for a licensed BC gas technician — not a DIY reset.
Severity
High — needs prompt professional attention
Gas safety first
This fault involves gas, combustion or venting. If you smell gas (rotten-egg odour), hear hissing, or feel unwell, leave the building immediately and call 911 or FortisBC at 1-800-663-9911 from outside. Do not flip switches or relight anything. For all other cases, switch the boiler off and call a licensed gas technician.
What the Bosch A4 error code means
A flue-gas or related safety device reported an abnormal condition.
On Bosch Greenstar and Singular series, A4 is a air-pressure / venting fault. It is a protective response — the boiler is telling you something needs attention, and the safe, lasting fix is a proper diagnosis rather than repeated resets.
Symptoms you may notice
- Lockout, often worse in wind, rain or cold snaps
- No heat or hot water
- Boiler tries to start, then faults during purge
Common causes of Bosch A4
Why this is not a DIY fix
Air-pressure-switch and flue faults are a combustion-safety issue: a blocked, disconnected or leaking vent can spill flue gases — including carbon monoxide — into your home. This must be inspected and corrected by a licensed gas technician, never patched or bypassed.
In British Columbia, gas work must be performed by a TSBC-licensed gas fitter. GasBoilers.ca — a CanroHeat division — sends Red Seal–certified, gas-licensed technicians who diagnose the true cause, repair it correctly, and verify safe operation before they leave.
How our technicians diagnose & repair it
- 1Inspect intake and exhaust venting for blockage, sagging or disconnection
- 2Test the air-pressure switch and its sensing tubes for blockage or condensate
- 3Check the condensate trap and flue for water back-up
- 4Verify safe combustion and draft with analysis before return to service
Related Bosch codes
Bosch A4 — FAQ
What does the Bosch A4 error code mean?
A flue-gas or related safety device reported an abnormal condition. On Bosch Greenstar and Singular series, this is categorised as a air-pressure / venting fault.
What causes Bosch A4?
The most common causes are: Flue-gas sensor fault; Venting restriction; Combustion fault. A licensed technician confirms which one applies before any part is replaced.
Can I fix Bosch A4 myself?
No. Air-pressure-switch and flue faults are a combustion-safety issue: a blocked, disconnected or leaking vent can spill flue gases — including carbon monoxide — into your home. This must be inspected and corrected by a licensed gas technician, never patched or bypassed. Repeatedly resetting the boiler without fixing the root cause can be unsafe and can cause further damage. Call GasBoilers.ca at 604-359-1081 for a licensed diagnosis.
Is the Bosch A4 error code dangerous?
It can be. This fault involves combustion, gas or venting. If you ever smell gas, leave the building immediately and call 911 or FortisBC at 1-800-663-9911 from outside. Otherwise, switch the boiler off and book a licensed technician.
How much does it cost to fix Bosch A4 in Greater Vancouver?
Cost depends on the root cause and any parts needed. We start with a flat-rate diagnostic, then quote the repair before any work begins — no surprises. Call 604-359-1081 for current pricing and same-day availability.
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Bosch showing A4? We'll fix it right.
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