Overview
F.70 on Vaillant EcoTEC Pro / EcoTEC Plus boilers means the boiler has detected an invalid Device Specific Number (DSN). The DSN is a configuration value used to pair the printed circuit board (PCB) to the specific boiler model and options. This fault commonly appears immediately after a PCB replacement (new or reconditioned) because the replacement board is generic and must be set to the correct DSN for that boiler. Severity: F.70 is a non-ignition/configuration fault that prevents the boiler from operating normally until corrected. It is not an immediate safety emergency like a gas leak, but the boiler will not provide heat or hot water while the fault is active. If the boiler was recently powered down or the PCB replaced and F.70 appears, it usually indicates a setup/configuration step was missed rather than a mechanical failure. However, repeated PCB failures or associated errors (for example F.61) can indicate underlying hardware issues that need professional attention. DIY vs professional: Some competent DIYers or experienced heating engineers can clear F.70 by entering the service menu and programming the correct DSN offset for the specific boiler model. However, you must be confident working with the boiler controls and understand the risk of changing service settings. If you are unsure, if the fault doesn’t clear after setting the DSN, if the PCB keeps failing, or if there are any gas-related concerns, you should call a qualified, registered heating engineer (Gas Safe in the UK).
Possible Cause: Invalid device specific number (DSN)
Troubleshooting Steps
Safety precautions:
1) Before working on the boiler, switch off the electrical supply at the isolator and, if you will be touching gas components, turn off the gas supply and call a Gas Safe engineer. Do not attempt gas valve or burner repairs yourself. 2) If you remove the boiler cover to inspect the PCB, ensure the unit is isolated from mains power and you are static-free; live work should only be done by a qualified engineer.
Initial checks a homeowner can do:
1) Simple reset: Try a boiler reset first using the reset button or turning the boiler off and on at the mains. Note the exact behavior and any other error codes (for example F.61). 2) Confirm history: Was the PCB recently replaced or the boiler powered off? F.70 commonly appears after PCB replacement. 3) Visual check: With power off, open the casing and visually check for loose connector plugs to the PCB, obvious burn marks, or disconnected cables. Do not touch wiring with the power on.
Specific diagnostic and fix steps (for experienced DIYers or engineers):
1) Confirm PCB compatibility: Verify the replacement PCB part number matches the boiler family or that you have the DSN offset table for that PCB (many EcoTEC boards share the same PCB and require DSN offsets). If you have a PCB with part number 0020132764, use the corresponding DSN offset table for your exact boiler model. 2) Enter service/installer menu: On many older EcoTEC models, you enter the diagnostics menus by holding the "i" and "+" buttons together. Navigate through the menus using the plus/minus controls. 3) Access installer level: Go to menu d97 and enter the installer code 17, then press and hold "i" to accept. This should open the installer-level settings. 4) Set DSN offset: Navigate to d93 (sometimes labelled DSM or DSN offset) and change the value to the correct DSN offset for your boiler model as per the Vaillant DSN table for your PCB. After entering the correct offset, hold "i" to accept the change. 5) Reset and test: After changing the DSN offset, press the boiler reset button or cycle the mains power off and on, then check whether the F.70 fault clears and the boiler returns to normal operation. It can sometimes take more than one attempt. 6) Factory defaults if needed: If changing the DSN does not clear the fault, you can set d96 = 1 to restore factory defaults, accept with "i", then re-enter the DSN offset at d93 and reset again. Be aware restoring factory defaults will erase any custom settings. 7) If F.61 appears after resetting or if the boiler still will not run: F.61 can indicate the PCB is faulty or there is a gas valve issue—stop and call a qualified engineer.
When to call a professional:
1) If you are not confident navigating service menus or working around electrical/gas equipment, call a Gas Safe registered engineer. 2) If the fault will not clear after correctly setting the DSN, or if the boiler shows multiple faults or repeated PCB failure, you must call a professional to diagnose and repair. 3) Any work involving the gas supply, gas valve, burner, or mains electrical connections must be carried out by a qualified engineer.
Additional notes:
1) Reconditioned PCBs sometimes arrive without DSN tables or instructions; obtain the correct DSN offset table for your boiler model and PCB part number before attempting changes. 2) Keep a record of any setting changes you make and the original values so a professional can review them if needed. 3) Avoid multiple power-cycling attempts if you suspect the PCB is damaged—repeated power events can further damage components. 4) No cost estimates provided here; consult local engineers for quotes if parts or service are required.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Pro / EcoTEC Plus.