A/C intermittently working.

[StevieST95]

Sometimes on startup it wont blow cold. I’ll try to restart my car and usually it’ll work but more than likely it won’t until I try my “remedies”.

 

[Dialcaliper]

This sounds suspiciously like the problem that was common on my old Honda Fit. On really hot days, the A/C would blow nice and cold most of the time, and then after a while peter out, blowing warm air, but I noticed the airflow out the dash vents also seemed to get weak. The hotter it was outside, the worse the problem would get.

It turned out that the evaporator coil was actually getting too cold and icing over, preventing most air from getting through and not enough coil exposed to cool the air blowing through resulting in warm, weak air from the vents (but the fan noise would sound normal). The cause turned out to be that the resistance of the thermistor that regulates the evaporator temperature would drift over time, and it would eventually read warmer than actual, so the A/C would run too cold and freeze the condensation on the coil instead of the water just dripping out the drain like it should. Icing can theoretically happen because of low refrigerant charge, but this is far less likely on modern electronically controlled A/C systems that it was on old A/C systems that use very simple Hobbs style pressure and thermostat switches

The “going over a bump” remedy is what made me think of this, which would knock ice off the coil, and running at lower RPM could result in a lower compressor speeds and pressure (less prone to overcooling).

You can check the resistance of the evaporator temp sensor, the test values are in the service manual. Fortunately, it looks like in this car it’s a very simple swap - in the Fit it was actually screwed into the evaporator coil, requiring the system to be evacuated to change it out, so the solution there was to put a resistor or potentiometer in parallel to correct the drift in resistance. May not be the cause, but its an easy place to start checking - here's a clip from the service manual. It appears to have a very large and detailed diagnostic section for the climate control system. You need to wait until the car has been sitting turned off for a while so that the coil stabilizes to ambient temperature. Helps to have a thermometer so you can actually check the cabin temperature in the car.

If you have Forscan, you can also ping and check for trouble codes in the IPC module (instrument panel cluster). These codes as well as the Body Control Module (BCM) and like 5 other independent computer modules in the car do not show up on something like an Accessport, which only reads OBD codes from the Engine Control Unit.

 

[StevieST95]

Thank you for the insight. After making the previous post I was looking up my concern and found some videos that would say that the evaporator temperature sensor would cause similar issues to mine. I just replaced it yesterday but it feels like that problem is still here. It got cool so I’m just waiting for this weekend to be hotter to see if it helped out any or if it definitively not work. I have the HVAC unit with Auto and heard that the cabin temperature sensor can also cause something similar.

 

[StevieST95]

Also I thought what I had replaced was the Evaporator temperature sensor and that it was only a two wire sensor like the one attached below.

 

[HAHAFiST]

Any updates on this post? I'm literally going through the same thing and it started happening on 90º-100º days over the last week.

 

[StevieST95]

Sorry for the late reply but the only luck that I have was replacing the cabin ambient temperature sensor. I also leave it on auto, recirculating and it seems to work.

 

[Unknown280]

This is probably a long shot trying to get an answer on this old thread but I’ve noticed some things throughout the years. (The issue still continues, I’ve just dealt with it.) If my air starts blowing warm I can either hit a bump or downshift and get my rpm’s above 3k and it’ll start blowing cold again. On the highway these two “remedies” will work until I hit the next bump or if I start cruising below 3k, same in stop and go traffic. I plan on replacing my radiator soon so I’ll check the kinked hose TyphoonFiST mention until then. I also have a buddy at a dealership with an A/C machine and we verified that it had the correct amount of Freon in it but just to be safe we vacuumed and refilled it to spec. I don’t know if any of this info is useful but I appreciate any help. Thank y’all 🙏🏽.

Im having the same problem, were you able to figure out what was the pproblem. I have taken my car to two different mechanic and they weren't able to figure out the problem. Any help would be appreciated.

 

[divinjup]

Just for the record, I have experienced this as well, especially city driving. Does not happen often, but getting up to highway speeds, or even restarting vehicle usually fixes it.

 

[Tnort]

Another chiming in. I was just sitting with the car idling, and all of a sudden the AC started blowing warm. Since then, the A/C wont work at all unless the motor is under load. My symptoms sound similar to what you guys are experiencing. I was assuming it had something to do with the clutch not engaging properly, but now I am not so sure...

 

[StevieST95]

Im having the same problem, were you able to figure out what was the pproblem. I have taken my car to two different mechanic and they weren't able to figure out the problem. Any help would be appreciated.

Yeah like I mentioned the only luck that I’ve had so far is replacing the cabin ambient temperature sensor. I replaced it late June and had mixed results. When I first replaced it I put it on Max AC and it cut out once and when I restarted my car it didn’t work at all. After that I put it on Auto and Recirculating and it’s been working fine.I haven’t tried other settings cause it’s too hot for me to even risk it 😁. Maybe when cooler temperatures come around I’ll try more settings but I need the ac during these hot times.

 

[akiraproject24]

My Focus was blowing moist warm then ice cold for a while before I dumped it for other reasons (DCT at 170k and steering rack failure) anyhow, after researching not knowing much about AC, I wanted to replace the Schrader valve and the dryer pack on the side of the condenser. Seemed like 2 items that would eventually need it

 

[Tnort]

Just a note

Another chiming in. I was just sitting with the car idling, and all of a sudden the AC started blowing warm. Since then, the A/C wont work at all unless the motor is under load. My symptoms sound similar to what you guys are experiencing. I was assuming it had something to do with the clutch not engaging properly, but now I am not so sure...

Update: It turns out my radiator fan is non-functional which was causing the no A/C issue. I did not discover that the fan wasn't working until I went for a little road trip in 90 degree weather. My commute is mostly country roads with very few stops, so the car never got hot enough to throw a light for over a week, lol. Now I get to figure out what component has failed, already crossed the fuse and relay off the list...

 

[JorJorBinks]

I hate to revive a dead thread! Sorry guys. 🤙

I read something on this issue and it could be the interior HVAC switches themselves. I haven’t been able to fix the issue myself, the AUTO trick doesn’t seem to work for my instance.

My car is experiencing AC cooling issues only while running the engine at city driving ranges. Highway and canyon roads are totally fine. Idling is also fine it will idle indefinitely with no interior cabin temp rising. Outside temp has been ranging from 75-100*f, engine and oil temps are all within spec 185-215*f coolant, 175-230*f oil. The fan works just fine and doesn’t seem to be shutting off in accordance with the AC temps changing.

I took it in to my local garage and they did a leak down test, and confirmed that I had the correct amount of refrigerant and no leaks.

The only thing I know for sure triggers the warm air problem is light throttle acceleration around town.

I have my AP reading the “AC Pressure” to me and in city driving the pressure drops to around 100ish. Then at idle it stays around 165-185ish. So the air conditioning pressure is dropping when I experience the warming of the air.

I’m pretty stumped by this one! I think it’s an electronics issue