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Fujitsu FO20R Series Installation Instructions Manual page 69

Variable speed r-410a heat pump outdoor units
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9.12 Service Analyzer Charts (cont.)
Bad pressure reading
Suction thermistor in wrong location
Poor system control
using an EXV
Bad suction thermistor
Improper superheat setting (less than 5°F [-2.8°C]) Adjust EXV SH setpoint dipswitches and validate
Moisture freezing and blocking valve
Dirt or foreign material blocking valve
Low refrigerant charge
Vapor bubbles in liquid line
High Superheat, Low
Suction Pressure
(superheat over
15°F [8.3°C])
Undersized EXV
Incorrectly sensing vapor line temperature
Suction thermistor incorrectly calibrated.
Vapor pressure measured incorrectly.
EXV is stuck
VSODC dc circuit failed
High superheat adjustment
9.0 DIAGNOSTICS & TROUBLESHOOTING
POSSIBLE CAUSE
Check transducer wiring.
Relocate thermistor.
Replace thermistor.
valve operation.
Recover charge, replace filter-drier, evacuate system,
recharge.
Recover charge, replace filter-drier, evacuate system,
recharge
Correct the charge.
Remove restriction in liquid line. Correct the refrigerant
charge.
Remove noncondensible gases.
Size liquid line correctly.
Replace with correct valve.
Verify suction thermistor resistance is correct and
properly attached and insulated to the vapor line.
Replace suction thermistor assembly.
After verifying lack of connectivity, replace the pressure
transducer or harness.
If harness has a short, replace harness
If gauge pressure measurement and the converted
voltage readings at the control are different replace
pressure transducer
If gauge pressure measurement and the converted
voltage readings at the control are the same, replace
the control.
Remove EXV from the system and purge with nitrogen,
replace filter drier, and recharge.
If EXV will not open, validate EXV wiring harness and
rotor resistance between the black wire and the
other 4 wires.
If none of the above rectifies the issue, replace EXV
and filter drier and recharge.
Verify 5Vdc to pressure transducer is present. If not,
replace VSODC.
Change the superheat offset dip switches to increase
superheat.
69

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