A.O. Smith fault code F21
Fault code F21 on an A.O. Smith water heater (including Innovo) means: Air pressure switch fault: Damaged wiring/open circuit. Reset the appliance once. If the code comes back, a part is usually faulty. You can check the water pressure and settings yourself; leave work on gas-carrying parts to a certified gas engineer.
After a safety alert (gas, flue gas, combustion), reset no more than once and call in a certified gas engineer. If you smell gas, call your national gas emergency number or 112 immediately.
Cause
Fault code F21 on an A.O. Smith water heater indicates: Air pressure switch fault: Damaged wiring/open circuit.
Fixing fault code F21: step by step
- Reset the appliance once. If the code comes straight back, do not keep resetting it.
- If the code comes back, the small hose to the air pressure switch is often dirty or the switch itself is faulty. Have it checked by a qualified heating engineer.
Applies to
A.O. Smith Innovo, IR 12-160, IR 12-200, IR 20-160, IR 20-200, IR 24-245, IR 24-285, IR 32-245, IR 32-285.
Matching parts
A.O. Smith parts are available from Instalpunt; shipping to EU countries from €12.95 (€16.95 for more distant countries), usually 3–5 working days.
This message does not point directly to one faulty part. These are the parts that most often fit this appliance:
- A.O. Smith ionisation pin after September 2009 € 43.49
- A.O. Smith glow plug for e.g. Innovo IR32-380L 0311990(S) € 96.49
- A.O. Smith fan adapter € 101.93
Fix it yourself or call an engineer?
Always consult the manual for your appliance and work safely. If in doubt or if the fault comes back, call in a qualified professional. A question about a part? Write to us at info@instalpunt.nl.
Frequently asked questions
What does A.O. Smith water heater fault code F21 mean?
Air pressure switch fault: Damaged wiring/open circuit.
How do I fix fault code F21 on an A.O. Smith water heater?
First check the cause described above. Then restart the appliance using the reset button. If the message comes back, a part usually needs replacing or the fault needs to be repaired by a qualified professional.
Which parts do I need for A.O. Smith fault code F21?
That depends on the exact cause. In the A.O. Smith parts overview at Instalpunt you will find the current price and stock for each part.
Can I fix fault code F21 myself?
You can reset the appliance and check things like the water pressure or settings yourself. Work on gas-carrying parts must be left to a certified gas engineer.
All A.O. Smith water heater faults and codes at a glance →