Citroën C4 Mk1 (2007) 1.6 NFU – Intermittent AC activation issue

[tenxin90]

Hello,

I have an intermittent HVAC/AC issue on a Citroën C4 Mk1 (2007, 1.6 petrol NFU, dual-zone automatic climate control).

Main symptom
• AC / blower system is intermittent:
? Sometimes the system does not start at all (no blower activation)
? When it does start, everything works perfectly:
? all blower speeds work normally
? cooling performance is normal
? no fluctuation or weakness
• Earlier in the fault history, the system would sometimes start normally but then shut off after a few seconds. Now it is more often a complete “no start”

Parts already checked / replaced
• Blower motor: tested externally ? OK
• Blower control module (resistor/transistor): replaced
? PSA reference: 6441.S7 / 9658084577
• Climate control panel (dual-zone automatic unit): replaced with used unit ? no change

• External relay has been installed in blower power circuit (previous repair work)
• Electrical measurement observed: ~6.5–6.6V on HVAC control/reference line (exact interpretation unclear)

• When the system is working, it is completely stable
• Failure is intermittent and “binary” (works perfectly or does not start at all)
• No gradual degradation in performance when active

Has anyone seen similar intermittent AC activation on C4 Mk1 where:
• blower and control module are known good
• but system sometimes does not receive/execute start command
I am trying to determine whether this is more likely:
• BSI/communication issue
• power/ground stability issue
• HVAC control signal issue
• or known wiring fault point

Thank you

[mickthemaverick]

Since you have an 'it does or it doesn't' situation I would go through all the connectors around the fuse boxes looking for corrosion or misplaced pins. It seems that you have an intermittent connection somewhere!

[ozvtr]

• External relay has been installed in blower power circuit (previous repair work)

In the BSM? Is power getting to the fan speed controller?