first check coolant level, add if require, J9 i" l; j+ T
FLUSH COOLANT - t6 t; R/ l) o/ l0 lCHANGE THERMOSTAT : m! r* G8 z+ ~& R* N
if still doesn't work, flush coolant again 7 T1 e2 I) L7 w$ f6 [
if still doesn't work, maybe change heater core(big job) ) J# e L- i r- K. l