The most common reason for this to happen is low engine coolant. If that is not the reason then my next check would be the coolant flow through the heater core that is supplied by the small radiator coolant lines that are routed to the firewall.
SOURCE: Heater blowing cold on 2005 Nissan Xterra
I live in Alaska and have had the same problem i just added more coolent to the radiator and it warmed up i installed a block heater and i leaked a bit and the coolent level got low and i would have no heat. I tryed to fill just the coolent resivoir and that dident work so i filled the radiator pumb full and that worked!! Hope this helps.
SOURCE: Nissan Altima no cool air blowing or hot air blowing
Hi!
It
appears we have an Air lock scenario and you will need to perform a
system Bleed.
Park
the vehicle on level ground, when cold remove coolant filler cap,
start engine and leave to idle, turn heater on full and blower to
max. When engine reaches operating temperature watch and listen near
coolant filler, keep clear as gurgling and hopefully a boil over
should occur. Top up with very warm coolant and wait as it may do it
again.
Check
for heat inside vehicle if warm replace coolant cap but keep an eye
on temperature gauge as the ~Air lock may have moved on from heater
matrix/core so proceedure needs to be carried out again from COLD.
Please
press the Blue button to appraise my FREE Efforts, Thank You!
Paul
'W' U.K
SOURCE: nissan dealer claims they replaced the heater pump
Yes, your Nissan Pathfinder has a heater pump. The heater pump circulates the hot coolant from your engine to your heater core(s). You have two heater cores if you have rear temperature controls. My guess is that your problem was that you had very little heat or no heat at idle. Attached is the page from the service manual on how to replace it.
SOURCE: 2005 nissan altima heater blows cold air when
Check your coolant level. If it is low, what you describe will happen. Water pump works faster as rpm's increase, managing to get hot coolant to heater core (highway speeds). At idle, pump is not working fast enough to get the coolant to the heater core.
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