Have a 1996 Maxima GLE with 297K. This winter I've noticed the heat is nowhere near as effective as it used to be. I used to have solid heat (to where the fan/blower would slow down [automatic climate control], or I'd turn it off) within the first few miles of driving. Lately the blower stays on high and only blows luke-warm air-- enough that I'm not freezing, but I really miss the HOT heat on cold mornings or when I have to defrost the windshield quickly.

I've verified with a scan tool that the thermostat is regulating properly. Coolant temp gets to 190F within a few miles and never drops below 185F even on the coldest days. Sitting idle for a long time, coolant temp will get up to 205F then fan kicks on as it should, bringing temp down to 195F quickly. So I don't think this is a cooling system issue. I've also pressure tested it; no leaks. Temp gauge reads right in line with what the scan tool shows.

What else should I be looking for?

 

When was the last time you flushed the system thoroughly and refreshed the 50/50 mix?

Sometimes backflushing the heater core can remove gunk accumulated where the heat exchange occurs. But te following is more likely...

Have you checked the coolant level?

Is your coolant reservoir empty and/or has the hose connecting it with the radiator been compromised?

If it's low, fill it while the car is facing uphill to get all the air out of the system as you top it off running and warm.

If all this is good, look at the blend door actuator.

 

I did a cooling system service over the summer- replaced both heater hoses going to the heater core, new OEM thermostat, new coolant, used Pentofrost A2 which is the Nissan OEM equivalent at the correct ratio. Did it because the service was due (no problems prior), I basically went over it with a fine tooth comb. Bled the system thoroughly using one of the Lisle funnels. Coolant level stays full in the reservoir, and system passes a pressure test @ 15 PSI. I had good heat after this service, well into fall when temperatures outside were plenty cold. It's only recently in the last few weeks that I've noticed the heat is nowhere near as hot as it used to be. I've checked all the boxes as far as cooling system is concerned. Can't say I've backflushed the heater core, but this car has had regular antifreeze changes (at least every 75k or so, if not sooner.)

Are the heater cores known to plug/gunk up on these vehicles, and are there other possibilities for this issue?

 

Heater cores usually don't get gunked, but there's always an exception. And you just removed that possibility this past Summer.

Now that the easier stuff is all confirmed.... Now for the harder news. LOL

The FSM has a section "HA" Go to page 70 and it will guide you to start a self diagnosis of the heating/cooling system in the dash. Most likely your Blend door actuator has passed. Mixes from cold to hot and everything in between. And yes they go. The self diagnosis will confirm this and then source and replace.

Heat will return! LOL