Noritz EZ Series & NCC199CDV Code 66
Bypass water servo fault. The bypass water servo is not functioning. This is the second motorised valve: it routes some water around the heat exchanger and blends it back in, which is what stops the outlet overshooting at low flow. Noritz's checks are the same as for code 65 — that the servo functions, and the connection at the valve — and the same power cycle to clear it.
Also displayed as: F66
Part replacementThe verdict
Replace a part
Same shape as code 65 and a different valve. The symptom to expect is water that runs hotter than the setting at low flow, because the blend that normally corrects it is not happening.
Typical cost: $0–$700
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the EZ Series & NCC199CDV — not averaged across the brand.
Source: Noritz Service Manual — NRC663 FSV, EZ98DV, EZ111DV, NCC199CDV (Rev. Oct 2017) (p. 32), read Sep 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Cycle the power, which is Noritz's documented reset for this code.
- If it returns, book a service call and say it is the bypass servo, code 66, not the main servo.
Code 66 on other Noritz series
Code 66 is a different fault on every other Noritz series that documents it, which is exactly why this page is written for the EZ Series & NCC199CDV alone:
Don't confuse this with
One other Noritz EZ Series & NCC199CDV code points at a water servo too. They are different components in different places, and ordering a part against the wrong one is the most common way this fault gets expensive: