Sime Boiler Error ALL 02: What It Means and How to Fix It

As temperatures begin to drop, the reliability of your home’s thermal comfort becomes an absolute priority. Finding the error code “ALL 02” flashing on the display of a Sime boiler is a situation that naturally causes concern, especially as it usually immediately interrupts both the domestic hot water supply and the circulation in the radiator circuit. This preventive lockout mechanism is one of the most common safety warnings we detect in homes during the peak heating season.

According to the troubleshooting guidelines detailed in the official Sime boiler instruction manual, this alphanumeric alert specifically warns of a sharp drop in water pressure within the system. For the equipment to offer optimal and safe performance, the closed-circuit pressure must be between 1 and 1.2 bar when the machine is on standby and cold. Strict control of these technical parameters is essential and aligns directly with the standards promoted by the Institute for the Diversification and Saving of Energy (IDAE) regarding efficiency and preventive maintenance for domestic thermal installations.

At JAG Alcaide, we operate as comprehensive climate control specialists throughout Barcelona. We tackle these types of mechanical and hydraulic imbalances using precise and definitive diagnostic methods. Understanding whether the pressure loss is due to a tiny, imperceptible drip in the circuit, an air lock requiring bleeding, or an internal fault in the expansion vessel is vital for providing truly safe heating solutions. Our technical approach is always based on protecting the long-term integrity of the system and restoring absolute comfort to your home.

What does the ALL 02 error mean on a Sime boiler?

The appearance of a fault code on your heating’s digital panel usually causes confusion, but the ALL 02 warning on Sime manufacturer models is, in fact, a very well-defined self-protection mechanism. This error code indicates that the boiler has locked out preventively due to a lack of water pressure in the primary heating circuit. Without sufficient water volume, the unit cannot guarantee safe heat transmission or correct circulation, so it halts operation to prevent major breakdowns in the heat exchanger.

For a boiler of this brand to work in optimal conditions, the cold circuit pressure must always be between 1.2 and 1.5 bar. If the indicator falls below the critical threshold of 0.5 bar, the water pressure switch detects the anomaly and immediately activates the safety lockout. It is crucial not to confuse this signal with the ALL 03 error, which refers to an overtemperature problem or a flue thermostat fault—an entirely different type of incident that requires checking the gas flue or the condition of the temperature sensors, rather than the water pressure.

Resolving this pressure issue may require different levels of technical intervention depending on the exact origin of the fault:

• Diagnostics and circuit bleeding: Useful for removing trapped air pockets and readjusting to the correct filling levels. The cost of this professional service usually ranges between €65 and €95.
• Replacement of the water pressure switch: If the measuring component is damaged and gives false readings even when the circuit is full, replacing it costs between €110 and €170.
• Repair or replacement of the expansion vessel: When the pressure fluctuates uncontrollably every time the heating is turned on, working on this component varies between €180 and €290.

Keeping a regular eye on the pressure gauge helps prevent the system from reaching this shutdown state and prolongs the lifespan of the installation.

How to fix the ALL 02 error step by step (Filling instructions)

Returning the necessary water to the heating circuit is a straightforward process that requires precision to avoid major problems caused by overpressure.

The first step is to locate the filling loop valve. On almost all Sime models, this small plastic tap (usually black or blue) is found at the bottom of the appliance, right where the water and gas pipes connect.

Once the tap is identified, handling must always be done with the boiler cold and switched off:

• Turn the valve slowly anti-clockwise.
• You will hear the sound of water entering the heating circuit.
• Keep a close eye on the boiler’s pressure gauge (the pressure indicator, which may be a physical needle or a digital display on the screen).

The goal is to bring the pressure to a range of between 1.2 and 1.5 bar. As soon as the needle reaches this optimal zone, close the tap firmly by turning it clockwise. It is extremely important not to exceed 2 bar of pressure when cold, as the pressure will naturally rise when the water heats up, which could trigger the safety valve due to excess load.

Once the correct pressure is restored, the ALL 02 error code usually remains active on the screen for safety. To clear it and reactivate the unit, turn the boiler’s main control dial to the “R” (Reset) position. Hold the dial in that position for about three seconds and then return it to the usual operating mode (winter or summer). The system will perform a self-diagnostic check, and the burner will ignite normally.

Causes for the pressure dropping again and the ALL 02 error persisting

Topping up the heating circuit seems like the most obvious solution when the indicator shows a low level, but the real challenge arises when, after a few hours or days, the pressure plummets again and the lockout code systematically reappears. This cyclical behaviour usually hides internal anomalies that require precise technical diagnosis.

Water leaks do not always manifest as a large, visible puddle on the floor. In many homes in Barcelona, the root of the problem lies in dried-out seals, worn lockshield valves, or radiator valves that drip almost imperceptibly. Because the pipes and radiators are hot, this tiny amount of water evaporates immediately, making it visually difficult to locate the leak while the system continues to lose pressure constantly.

The expansion vessel is responsible for absorbing the increased volume of water as it heats up. If the vessel’s internal membrane loses its air charge or becomes punctured, the circuit pressure spikes rapidly when turning on the heating. To prevent structural damage, the three-bar safety valve opens automatically and expels the excess water. Once the boiler cools down, the pressure drops below the minimum, causing the lockout again.

The pressure switch acts as the boiler’s watchman, mechanically measuring the internal circuit pressure. Over time, this sensor accumulates sediment or suffers wear on its electrical contacts, preventing it from sending the correct signal to the printed circuit board. Even if the installation’s actual pressure is adequate, a faulty pressure switch will register a zero reading.

Limescale build-up, very common due to the hard water in the Barcelona metropolitan area, often prevents the filling valve from closing completely tight, causing instability and water backflows that alter the boiler’s proper functioning.

When is it necessary to call a professional heating engineer?

The ALL 02 error on a Sime unit is usually temporarily resolved by restoring the water pressure, but there are scenarios where the intervention of a specialist becomes essential to avoid major breakdowns. The clearest indicator that the problem requires professional attention is when the pressure repeatedly drops to zero within a few hours or days of topping up the heating circuit.

Ignoring this constant water loss and simply systematically topping up the system carries serious risks. The continuous introduction of fresh water brings new oxygen and minerals into the closed circuit, which accelerates the internal corrosion of the boiler’s metallic components and clogs the radiators with sludge. Furthermore, this behaviour can lead to hidden dampness in walls or floors, and puts critical strain on the circulation pump, which may end up burning out its motor due to dry running or the persistent presence of air in the system.

To solve the anomaly at its root, a qualified engineer must perform a technical diagnosis of the appliance’s internal components. This task involves safely removing the casing to check the condition of the expansion vessel’s membrane, verifying if the three-bar safety valve is leaking due to limescale build-up, or evaluating the tightness of the primary heat exchanger. In any of these situations, worn parts must always be replaced with official Sime spare parts, ensuring that the unit recovers its optimal levels of efficiency, performance, and operational safety.

Restoring the water pressure is, in the vast majority of cases, a sufficient measure to return a heating system locked by this warning back to normal. The ALL 02 code on Sime boilers acts as a highly effective self-protection mechanism, as it immediately stops the unit’s operation before a lack of flow can cause critical overheating in internal components.

Replenishing the water level via the bottom filling loop is a basic maintenance task that the user themselves can perform in a few minutes. However, the situation requires a different approach if this fault manifests recurrently. A boiler that constantly loses pressure is warning of a structural anomaly in the closed circuit. The origin of the imbalance is usually found in hidden micro-leaks in the installation, a pressure switch failure, or a rupture in the expansion vessel’s membrane.

Limiting yourself to repeatedly topping up the circuit is a serious mistake; this practice constantly introduces new water containing limescale and oxygen into the pipes, accelerating the internal corrosion of the entire system. Faced with a scenario of continuous loss, specialist intervention is indispensable. A professional diagnosis and circuit bleed, the typical cost of which ranges between €65 and €95, allows the exact source of the leak or sensor failure to be located, thereby avoiding breakdowns that require the replacement of much more expensive parts in the short term.

At JAG Alcaide, we are experts in the precise diagnosis and comprehensive repair of boilers and heating systems. Our technical team guarantees fast, effective interventions aimed at prolonging the lifespan of your equipment with total safety. We offer professional coverage for climate control solutions throughout Barcelona and its metropolitan area. Trust our specialists to restore comfort to your home and ensure the maximum energy performance of your installation.