Thank you for your help. I tired to replace T8611G1004 (control works but display almost gone) with RTH7400 for heat pump.

T8611G1004 wired as:
R - red
C - blue
Y - yellow
X2 - white
G -green
w2 - white
O - red

Wired RTH7400 as
C - blue
G - green
Y - yellow
O/B - red (was O)
R - red (jumper to RC)
AUX - white (was W2)
E - white (was X2)

When turned system to on, room temp was higher than set temp. System started to work but produced heat. What went wrong?

That's one old thermostat. Not knowing what equipment you have -- make sure the thermostat is setup for heat pump. Some of the older heat pumps energize the reversing valve in heat mode. The newer system will energize the reversing valve on the heat pump in cooling mode. The thermostat will have to be setup for your system.

Thank you, T-Top. I changed 170 to 7 and now it's cooling down. Should have read troubleshooting. :)