Check that there isn't water around the water heater first, which would mean that turning it off and closing the water valve would be a good idea. Then, check the fuse box to see if the power has tripped--that is if it is an electric water heater.
THERMOCOUPLE or defective gas valve
Low coolant level? Cooling system airbound? Bad thermostat? Water pump not circulating coolant? Heater control cable not working?
Have you turned the control from Pilot to On? Have you increased the temperature setting?
fan motor could be bad
By reading the directions that come with each heater as each one is slightly different
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you have a bad heater core
Drain the water from your 1988 Chevy cooling system. Remove the heater core water supply hoses. Remove the heater core retaining bolts. The heater core will come out.
Begin by removing the water supply lines from your Nissan heater core. Remove the heater core retaining bolts. The heater core will come out.
More than likely you will not have heat being the water wont be circulating through the heater core , When the hot water passes through the heater coil the air passing through the heater coil makes heat blow out the vents when you have selected a heat setting on your controls.
Electricity tends to prefer paths of least resistance, as does water and air.
Installation instructions come with tankless water heater, but would recommend someone experienced enough to install.