Bosch error indicators generally fall into a handful of categories, and understanding the category helps more than memorizing individual codes.
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Bosch gas boiler error indicators generally fall into a small number of categories: ignition failure, flame loss, venting or exhaust restriction, water flow or pressure faults, over-temperature protection, and sensor faults. The exact code shown and its precise meaning is specific to your model, so the owner's manual for your unit is the accurate reference, and diagnosing the underlying cause requires a licensed technician with test equipment rather than a general code lookup.
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Bosch, like most boiler manufacturers, uses different fault indicator schemes across its various boiler ranges and model years, and the same displayed code can mean different things on different models. Rather than publishing a list of specific numbers that may be outdated or wrong for your exact unit, this guide explains the categories of faults that trigger an error indicator on most modern gas boilers, so you understand what kind of problem you are dealing with while your manual or a technician confirms the specific meaning for your model.
Ignition faults occur when the boiler attempts to light the burner but does not establish a flame within its allowed attempt window, which can stem from a gas supply issue, an igniter or electrode problem, or a blocked burner. Flame loss faults happen after ignition succeeds but the flame sensor stops detecting a stable flame shortly after, often linked to gas pressure, a dirty sensor, or a draft issue.
Venting and exhaust faults are triggered by a pressure switch, flow sensor, or fan monitor detecting restricted or abnormal airflow through the intake or exhaust, which can point to an outdoor terminal blockage, an internal fan issue, or a flue obstruction. Water flow and pressure faults cover both low system pressure and restricted circulation, which the control system monitors to prevent the heat exchanger from overheating. Over-temperature faults are a protective shutdown when a temperature sensor detects the water is hotter than it should be, often tied to restricted flow. Sensor faults themselves, meaning the sensor's signal is implausible or missing entirely, point to a wiring or component issue rather than the condition the sensor is meant to measure.
| Category | What it protects against | Common underlying cause |
|---|---|---|
| Ignition | Repeated failed attempts to light the burner | Gas supply, igniter, or burner issue |
| Flame loss | Continued operation without a confirmed flame | Gas pressure, dirty sensor, draft disturbance |
| Venting/exhaust | Operating with restricted or abnormal airflow | Blocked terminal, fan issue, flue obstruction |
| Flow/pressure | Overheating from inadequate circulation | Low pressure, closed valve, pump issue, sludge |
| Over-temperature | Water exceeding safe temperature | Restricted flow, sensor or control fault |
| Sensor fault | Making decisions on bad sensor data | Wiring fault or failed sensor component |
An error code tells you which protection was triggered, not why it was triggered, and the same code can have several different underlying causes. A technician confirms the actual cause using combustion analysis, pressure testing, and component checks, commonly documented as manufacturer-specified diagnostic steps for that model, verify against the manual for your specific unit. Treating a code lookup as a substitute for that diagnosis risks addressing the wrong thing, resetting the boiler repeatedly, or missing a venting or combustion issue that has real safety implications.
Safety note
If you ever smell gas, hear a hissing sound near the unit, or notice soot, a burning smell, or headaches and drowsiness near the appliance, leave the home immediately and call FortisBC at 1-800-663-9911 from a safe location before doing anything else. Any work on gas piping, venting, or combustion components must be carried out by a Technical Safety BC licensed gas fitter.
The owner's manual that came with your model is the accurate source, since code meanings differ between Bosch boiler ranges and model years. A technician can also confirm it on site along with the underlying cause.
A single reset after checking obvious items like pressure is reasonable, but resetting repeatedly when the same code returns is not a fix and can mean repeated failed ignition attempts, which is worth avoiding.
No, fault indicator schemes vary across different Bosch boiler ranges and model years, which is why this guide describes fault categories rather than specific numbers.
Yes, many codes relating to flow, pressure, or venting are actually caused by something in the wider heating system or venting path rather than a failed boiler component, which is why a full system check matters, not just an internal boiler inspection.
Yes, since the code only indicates which protection triggered, not the root cause, and an apparently minor code can sometimes point to a more significant underlying issue like restricted flow or a venting problem.
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