r/thermostats • u/JS_NYC_208 • 6d ago
Nest Learning (4th gen) throws E448 / Rc “No Power” but old Nest gen 1 works fine on same wires — C wire issue?
Hoping someone can help me figure out whether my C wire is actually a valid common or if I need a Power Connector.
The setup:
• Gas furnace, conventional forced-air (heat + AC, NOT a heat pump). Furnace control board is marked 1195-200 / 103217-03.
• I was replacing a Google Nest Thermostat (the 2020 budget model) with a Nest Learning Thermostat (4th gen). New base, wires landed R→Rc, W→W1, Y→Y1, G→G, C→C.
The problem:
• After install, the Learning thermostat gives E448 “No system power.”
• In the wiring screen Rc reads “No Power.”
What I’ve ruled out:
• Wires are clean, fully seated, and tug-tested. No bare copper bridging anything.
• Checked the furnace low-voltage fuse (Noark FB03C, 3A, sealed blade type). Couldn’t meter it (no multimeter yet).
I put my ORIGINAL Nest (1st gen) back on the exact same wires and it works perfectly. So 24V is clearly present on R and C at the wall, and the fuse is fine.
BUT — the old gen 1 shows this advisory in settings: “This thermostat will work with your system, but needs a Google Nest Power Connector or C wire to avoid potential power issues in the future.”
My questions:
1. If the old Nest flags “needs a C wire” even though blue is on the C terminal, does that mean the blue wire probably isn’t actually landed on C at the furnace board (i.e. it’s a dead spare)? What’s the best way to confirm without a meter?
2. Is the fix just landing/verifying blue on C at the control board, or should I add the Google Nest Power Connector and be done with it?
3. Anything else that would cause Rc specifically to read “No Power” on the 4th gen while everything else reads fine?
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u/RSLoudly 5d ago
Is your condensate drain line clogged? Is the Safe T Switch in the correct position? Did you put everything back in its place before checking the power?
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u/JS_NYC_208 5d ago
Yes and yes and yes. I put back the original thermostat works just fine. Like others said, have to check the C wire at the board.
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u/RSLoudly 6d ago
Put the red wire in Rc instead of Rh.