Overview
P.00 on a Vaillant EcoTEC Plus is not a fault code in the usual sense but a test/purge program mode (often called the pump purge). When the boiler is put into P.00 it runs the pump and internal valves in a controlled sequence to purge air from the heating circuit and hot water circuit. Technicians run this during commissioning, after system work or after bleeding radiators to remove air locks and re-establish correct water circulation. This mode appears when it has been deliberately started from the control panel (or by a service engineer). If you see P.00 and the boiler is simply carrying out a purge, the severity is low — allow the program to complete or cancel it if it was started by mistake. If P.00 appears unexpectedly and is accompanied by loss of heating/hot water, other fault codes, persistent pressure loss, or if the boiler will not return to normal operation when the purge finishes or you try to exit it, that suggests a separate fault (electrical, sensor, pump or valve issue) and a qualified Gas Safe engineer should be called. Homeowners can run and cancel the purge and perform basic bleeding and pressure checks, but anything involving gas, internal valves, sensors or wiring must be left to a professional.
Possible Cause: The heating circuit and hot water circuit are purged
Troubleshooting Steps
Safety precautions:
- If you are unsure at any point, stop and call a Gas Safe registered engineer. Do not attempt gas or electrical repairs yourself.
- Keep children and pets away while you work. Beware of hot surfaces and hot water from radiators or the domestic hot water tap. Wear gloves and eye protection if draining water.
- Do not open the boiler casing or touch internal components unless you are qualified. Isolating the mains power can be done for a reboot, but do not interfere with gas isolator valves or internal gas parts.
Initial checks a homeowner can do:
1. Look at the display and note the exact message. If it clearly shows P.00 (P0), that means the purge program is active. Check for any other simultaneous fault codes.
2. Ask any household members or a recent engineer whether a purge or service was started deliberately.
3. Check boiler water pressure on the pressure gauge. Typical pressure is about 1–1.5 bar cold. If pressure is very low (<0.7 bar) the boiler may lock out or the purge may not complete correctly.
4. Check that external heating isolation/service valves (if accessible) are open and radiator thermostatic valves are not closed. Make sure the gas isolator cock to the boiler is open (do not attempt to adjust it if unsure).
5. Check radiators for air: some may be cold at the top indicating air to be bled.
Steps to safely cancel or let the purge finish:
1. If the purge was started by mistake and you want to stop it, try pressing the boiler reset/fault-clearance key for one second. On many EcoTEC models pressing reset will return the display to the normal screen once the purge stops.
2. If reset does not exit P.00, switch the boiler off at the front panel or the external isolator for 30 seconds and switch it back on to reboot. This will often clear test modes started accidentally.
3. If the purge is legitimate and in progress, the safest option is to allow it to complete. It should automatically return to normal operation when finished.
If the purge was started to remove air or after system work (bleeding and topping up):
1. Manually bleed radiators starting at the lowest radiator and working to the highest. Use a radiator key, capture water in a cloth/container, and re-pressurise the boiler to the correct level if pressure drops.
2. After bleeding, run the heating and observe radiator heat-up; check for leaks and for stable pressure.
3. If radiators remain cold or there are signs of a pump not running (no circulation noise) the problem may be pump failure, blocked pipework, closed valves or air locks that the purge cannot clear.
Diagnostic steps if P.00 won’t exit or normal heating/hot water does not resume:
1. Note any additional fault codes or symptoms (no ignition, low pressure, F.xx codes). These indicate separate faults that need an engineer.
2. If pressure repeatedly falls after purging, inspect visible pipework and radiator valves for leaks. Do not dismantle boiler pipework yourself.
3. If the boiler shows temperature sensor or flow faults, or if the pump appears not to run during purge, do not continue to attempt internal repairs — these are likely electrical or sensor issues.
When to call a professional (must-call situations):
- P.00 won’t clear or the boiler will not return to normal operation after cancelling or rebooting.
- You see additional fault codes, flame or ignition faults, persistent pressure loss, or no circulation after the purge.
- There are signs of leaks, blocked condensate, or the pump does not run and radiators stay cold.
- You need service-valve operation, replacement of sensors (NTC), pump, wiring or PCB work, or any gas-side intervention.
Final note: Running the purge and basic radiator bleeding and pressure topping are acceptable DIY tasks if you are comfortable and follow safety steps. Anything that involves opening the boiler, changing gas components, electrical wiring or internal sensors must be carried out by a Gas Safe registered engineer. If in doubt, call a professional to avoid safety risks and further damage.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Plus Gas Boiler.