670 Three-way valve fault

What the control is telling you

The 3-way (DHW/CH) valve is in the wrong position, stuck, or the PC Board cannot communicate with it.

Applies to: I-Series Plus Combi boilers — 150K BTU and smaller and 175K BTU and higher

Likely causes

• Loose, broken or damaged 3-way valve wiring
• Wet harness or connectors
• Poor CH system water quality
• Valve stuck or mispositioned
• Failed 3-way valve assembly

Test and repair steps

1. Inspect the 3-way valve harness back to the PC Board for loose, broken or damaged connections and make sure it is dry.
2. Check central heating system water quality.
3. Warning: Live-voltage testing is dangerous and should only be done by qualified technicians using proper precautions.
4. Read the valve position in performance data #15 (0 = mid, 1 = DHW, 2 = CH).
5. At CN12, grey pin #16 to brown pin #18: under 1 VDC when fully open or closed, 4–6 VDC in mid position. Orange pin #6 to grey pin #16 should be 11–14 VDC.
6. At CN12, pink pin #4 to red pin #3 and white pin #2 to blue pin #1 should each read 40–60 Ω.
7. Replace the 3-way valve assembly if readings are out of spec or the fault remains.

Models that show this code

8 models in I-Series Plus 150K and smaller, I-Series Plus 175K and higher

Other codes on the same units

• 610 Combustion fan motor speed fault
• 631 DHW recirculation pump – insufficient flow
• 651 Water flow control valve fault
• 661 Bypass valve fault
• 681 DHW supply temperature abnormality
• 700 PC Board fault
• 710 Gas solenoid circuit – valve position mismatch
• 720 Flame rod fault