Overview
P.01 on a Vaillant EcoTEC Plus is a test-program fault: it means the boiler reported an error while running the maximum-load (full output) test routine. On older EcoTEC models the P-modes are commissioning/diagnostic programs (P.00 pump purge, P.01 maximum load/high flame test, P.02 minimum load/low flame, etc.). The boiler has detected that it could not complete the maximum load sequence correctly — this can be caused by problems with gas delivery, ignition/combustion regulation, mass-flow sensing, fan or flue issues, safety limit trips, or electronics. Severity is moderate to high. This code does not by itself mean imminent danger, but it does indicate the boiler cannot confirm safe, correct operation at full output. Because the underlying causes involve gas, combustion and safety systems, most fixes require a qualified Gas Safe engineer. Homeowners can perform a few simple checks and a safe reset, but should not attempt internal or gas-related repairs. If you see P.01 while a serviceman is actively using P-modes, it may simply be a failed test during commissioning and the engineer will rerun diagnostics. If it appears unexpectedly during normal use and does not clear after basic checks, arrange for a Gas Safe registered engineer to investigate and repair.
Possible Cause: Maximum load test program error
Troubleshooting Steps
Safety first
1) If you smell gas, hear a hissing leak, or anyone has symptoms of carbon monoxide (headache, nausea, dizziness), leave the building immediately, call your gas emergency number and emergency services. Do not re-enter.
2) Do not open the boiler casing or attempt any internal repairs. Gas and combustion components must only be handled by a Gas Safe registered engineer.
Initial homeowner checks (safe, simple checks you can do)
1) Note the display exactly (P.01) and any other fault codes or lights. Write down when it started and what you were doing (e.g. heating on, hot water demand).
2) Try a single reset: press the boiler Reset button once, or switch the boiler off at the front panel and back on after 10 seconds. Do not repeatedly reset more than 2–3 times.
3) Check mains power: ensure the boiler has power (other appliances on same circuit working) and any fused spur is OK.
4) Check boiler water pressure on the pressure gauge. It should normally be around 1.0–1.5 bar when cold. If pressure is very low (below ~0.7–1.0 bar) follow the manufacturers’ instructions to top up. Low system pressure can cause lockouts or test failures.
5) Check gas supply: confirm other gas appliances (hob, gas fire) work. If they don’t, contact your gas supplier. If you suspect a gas supply issue, do not attempt repair yourself.
6) Check external controls: make sure room thermostat or programmer hasn’t turned heating off; confirm demand for heating/hot water is set.
7) Check visible condensate and flue: ensure the external flue exit is not blocked and condensate drain/pipe is not frozen or blocked (visible outside). Clear obvious external blockages if safe to do so.
Specific diagnostic steps you can safely do or observe (do not open the boiler)
1) If you (or someone) deliberately put the boiler into P-modes, exit by pressing Reset or power-cycling the boiler. On older EcoTEC models P-modes are entered by holding the + button while switching on; exiting is done with Reset or switching off.
2) If you can safely observe the boiler when you attempt a restart: watch for ignition attempts, listen for the fan and pump, and note any additional error codes that appear (F codes). Take photos of the display for the engineer.
3) Do not attempt to run the P.01 maximum-load test yourself unless you are a qualified service engineer. That test forces the boiler to run at high output and involves gas/combustion adjustments only a trained technician should perform.
When to call a professional (and what to tell them)
1) If P.01 persists after a single reset and the basic checks above, contact a Gas Safe registered engineer. If the code appeared during normal use, or you have loss of heating/hot water, call promptly.
2) Tell the engineer the exact fault code (P.01), when it started, any other codes shown (F.xx), boiler model and serial number, whether you attempted a reset, boiler pressure reading, and whether other gas appliances are working.
3) Advise the engineer if you noticed anything unusual (blocked flue, frozen condensate, gas smell, etc.). Send photos of the display if possible.
What the engineer will check and why (for your information)
1) Re-run the P.01/maximum-load diagnostic safely and interpret the test results.
2) Check gas supply pressure and meter, inspect and test the gas valve and mass-flow sensing components (mass flow sensor / gas flow regulation), ignition electrodes, and flame detection.
3) Inspect fan and flue system, check flue gas temperature limiter and safety cut-outs, verify NTC sensors and wiring, and check the PCB for related faults.
4) Perform combustion analysis and make safe adjustments or replace faulty parts as required, then re-test the boiler.
Final notes
1) Do not repeatedly reset the boiler: frequent resets can mask an ongoing fault and may damage components. If a single reset does not clear the fault, arrange for a professional visit.
2) Any work on gas components, the gas valve, or internal electronics must be carried out by a Gas Safe registered engineer. Record the fault code, take pictures, and provide these to the engineer to speed up diagnosis.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Plus Gas Boiler.