Poor "Terminating" Sensor Contact; Undercharge - Goodman M Series Installation Instructions Manual

If voltage is registered at the coil, tap the valve body lightly
while switching the system from HEATING to COOLING,
etc. If this fails to cause the valve to switch positions,
remove the coil connector cap and test the continuity of
the reversing valve solenoid coil. If the coil does not test
continuous - replace it. If the coil test continuous and 24
volts is present at the coil terminals, the valve is inoperative
- replace it.
Figure 16: Reversing Valve

Poor "Terminating" Sensor Contact

The unit's defrost terminating sensor must make good
thermal contact with the outdoor coil tubing. Poor contact
may not terminate the unit's defrost cycle quickly enough
to prevent the unit from cutting out on high discharge
pressure.

Undercharge

An undercharged heat pump on the heating cycle will
cause low discharge pressure resulting in low suction
pressure and frost accumulation on the outdoor coil.
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