Overview
S.98 on a Vaillant EcoTEC Plus regular boiler is a status code that indicates the boiler is running a flow/return sensor check because it has detected a problem with water circulation or the temperature readings across the heat exchanger. In plain terms the boiler believes the flow and return temperatures are not behaving as expected (for example a very fast temperature rise or no expected temperature change) so it will block demand or shut down to protect the appliance. Common causes are poor circulation (blocked or air-locked pipework or pump), closed isolation valves under the boiler, debris or sludge in the system, or a fault with one of the NTC temperature sensors or their wiring. Severity ranges from an inconvenience (no heating/hot water) to a safety shutdown; because the boiler can lock out to prevent overheating or damage, this is not something to ignore. Some basic checks and resets can be done by a competent homeowner, but most diagnostic and repair work—especially any test or replacement of sensors, wiring, the pump, or internal components—should be carried out by a Gas Safe registered engineer.
Possible Cause: Flow return sensor check is running demand is block
Troubleshooting Steps
Safety precautions:
- If you smell gas, stop immediately, turn off the gas supply at the mains isolator and call the gas emergency number. Do not attempt repairs.
- Switch the boiler off at the programmer and isolate electrical power at the fuse or isolator before removing external covers for any inspection. If you are not comfortable doing this, do not open the boiler.
- Beware of hot surfaces and hot water when feeling pipes; allow the system to cool before touching internals.
Initial homeowner checks (safe, non-invasive):
1. Check the boiler display for exact status and try a single reset (follow the manual’s reset button procedure). If the code returns immediately or repeatedly, stop and proceed to the checks below.
2. Check system pressure on the pressure gauge. It should typically be around 1.0–1.5 bar cold. If pressure is low, repressurise the system using the filling loop to the correct pressure, then reset the boiler and see if S.98 clears.
3. Ensure any isolation valves on the flow and return pipes under the boiler are fully open.
4. Listen for the pump when the boiler calls for heat (you may hear the pump run or vibration). If totally silent when heating is demanded, the pump may not be running or is seized.
5. Bleed a couple of radiators (top-floor first) to release trapped air which can cause circulation problems and then check pressure again.
6. Feel the boiler pipework: when CH is called, the flow pipe should get warm and the return slightly cooler; if both remain cold or the flow is much hotter than the return instantly, that indicates a circulation or sensor issue.
Specific diagnostic steps and fixes (when to call a professional):
- If basic checks above do not clear the code, call a Gas Safe registered engineer. Explain S.98 and the checks you have done.
- The engineer will safely remove the boiler cover and test the flow and return NTC sensors and their wiring with a continuity tester and resistance meter. Faulty sensors, damaged harnesses, or a PCB fault may need replacement.
- The engineer will confirm pump operation and may test pump electrical supply and rotation. A seized or degraded pump, or failed pump seals, will require repair or replacement.
- They will inspect for blockages or airlocks inside the appliance (filter, heat exchanger, plate heat exchanger, mass flow areas) and in the wider heating system. If sludge or debris is present they may recommend a powerflush, cleaning of the heat exchanger, and fitting or cleaning the system filter/strainer.
- If temperature rise or STB (safety temperature limiter) faults are suspected, the engineer will check and clean or replace the STB and inspect the flue and fan for blockages or recirculation.
- If wiring harness, coding resistor, or PCB communications are implicated, the engineer will trace and repair/replace these components.
Final notes: do not attempt to work on gas, ignition, or internal electrical components yourself. S.98 can be caused by simple, homeowner-fixable issues (low pressure, closed valves, airlocks) but persistent or repeat faults require a qualified Gas Safe engineer to diagnose and repair safely. Contact a Gas Safe engineer and provide them with the S.98 code and a summary of the checks you have already completed.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Plus Regular Gas Boiler.