I have a 2001 rover 45 1800 impression and the heater only works on 3&4 settings , is it the dial or the heater that is faulty and how do i fix it

The most common reasons an INFINITI I35 heater isn't working are a broken heater blower motor, a problem with the thermostat, or a failed heater blower motor resistor.

INFINITI I35 Heater Is Not Working - RepairPal.com

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sounds like you need a new blower resistor, its under the dash on pass. side. if it goes bad your blower will only work on high. its fairly easy to change

SOUNDS LIKE THE SWITCH & OR THE BLOWER MOTOR RESISTOR, ARE THE PROBLEM WITH THE LOWER SPEEDS. SPEED 5 MAY HAVE ITS OWN FUSE, REFER TO OWNERS MANUAL FOR INFORMATION ABOUT THE HEATER/AC FUSE.

blower motor resister

You may have a faulty fan switch. Had mine replaced when only #1 and #4 were working and it fixed the problem. Unfortunately, there may be more issues, as a year later the switch is failing again. May be some kind of feedback that is causing it to fail.

Hey,
I'll assume for now that the blower motor is working.
Now, there are a couple of things things to check.
1. Easiest thing to check is the coolant level. If the coolant is low, your heater will not give any warm air. Hopefully this is the problem as it's the easiest fix.
2. The thermostat. When the engine and coolant are hot, your top rad hose should be hot as well. See if the rad hose is hot to the touch. If it isn't, your thermostat might be faulty. It's a fairly easy and cheap fix as well.
Good luck.

HI, its the resistor switch. it is malfunctioning. the mechanism has failed. replacing the switch will correct this issue.

Faulty switch.Change.

There is a vacuum valve that controls the heater pipe control valve.Check if the pipe has not perished or if the vacuum switch is not faulty

Did you replace the fan the blows the air into the car or just the heater? It could simply be a blown fuse.