Check that the water/coolant level is up to the MAX mark on the expansion/resovoir bottle, if it's not, the water won't circulate around the heater, also check that the valve is operating that opens the hot water to the heater, it is situtated on the bulkhead/firewall, behind the engine, get someone to operate the hot/cold control knob while you watch the valve, the cable sometimes comes off!
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