Carrier TP-NHP - Manual (Page 48)

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E3 − If AC/HP Control cannot properly read outdoor temperature, and it is
needed for proper operation, display will indicate “−−” in the outdoor
temperature location.
E4 − If AC/HP Control’s internal memory fails, E4 will be displayed. Replace
AC/HP Control.
Table 3 can be used as a troubleshooting tool for determining which outputs will
be active for a particular configuration and each operating mode.
When replacing a failed component such as an equipment control module or a
display module, the installer should replace both parts as a matched set. It is very
easy to accidentally mix an A/C or HP display with a PAC equipment control
module. When this happens, some functions appear in the setup while others do
not. This can be very confusing to troubleshoot. Verify that both parts have the
same model and serial number when troubleshooting the thermostat.