Heater issues

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mortten

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Not a Yukon but a 2014 Sierra. Guessing mostly the same. Has auto HVAC. Start the truck and after warmup heat works fine. After about 15 minutes air flow drops to almost nothing. I threw my scanner on it. No codes. Shut it down and let it sit 10 minutes and it works again after start up. Looking at some live data the evap temp sensor is stuck on 156 degrees. Playing around I find if after it acts up if I switch it to manual and turn off the AC button it works fine on manual. If the AC button is on it doesn’t. I don’t have a scope so I can’t check the sensor signal but in my mind that improper reading is causing the module to shut some things down thinking an evaporator overheat situation. What say ye?
 

Joseph Garcia

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Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

I cannot personally assist you with your issue, but @swathdiver and other members of this Forum much more knowledgeable than me in this area will chime in.
 
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mortten

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Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

I cannot personally assist you with your issue, but @swathdiver and other members of this Forum much more knowledgeable than me in this area will chime in.
797BFCE2-CA40-4B9A-A8D6-E56636002F69.jpeg
 

Doubeleive

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Not a Yukon but a 2014 Sierra. Guessing mostly the same. Has auto HVAC. Start the truck and after warmup heat works fine. After about 15 minutes air flow drops to almost nothing. I threw my scanner on it. No codes. Shut it down and let it sit 10 minutes and it works again after start up. Looking at some live data the evap temp sensor is stuck on 156 degrees. Playing around I find if after it acts up if I switch it to manual and turn off the AC button it works fine on manual. If the AC button is on it doesn’t. I don’t have a scope so I can’t check the sensor signal but in my mind that improper reading is causing the module to shut some things down thinking an evaporator overheat situation. What say ye?
what does the headliner air temperature sensor show?
 
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mortten

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what does the headliner air temperature sensor show?
Found the repair tree on Mitchell Pro Demand. I’ll run through it tomorrow. Most of it is checking the sensors with a scan tool against a thermometer. They want everything within 10%. One step is checking high side pressure against scan tool parameters. Guess I’ll have to dig my gauges out too.
 
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mortten

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what does the headliner air temperature sensor show?
No sensor in the headliner. It on the on the dash in combination with the sun load and ambient light sensor. Thermometer shows it to be within scan tool parameters. Seems like an odd place. Everything checks except the evaporator temp sensor. It’s only $30 so I ordered one. I’ll install it and see what happens. It’s possible the HVAC module isn’t sending a proper signal. I may pick up one of those pocket scopes so I can check stuff like that. From reading even though the sensor is showing incorrect temp no code is being set because it’s not out of the range the module is looking for which is -60 to +400 degrees F.
 
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Doubeleive

Wes
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No sensor in the headliner. It on the on the dash in combination with the sun load and ambient light sensor. Thermometer shows it to be within scan tool parameters. Seems like an odd place. Everything checks except the evaporator temp sensor. It’s only $30 so I ordered one. I’ll install it and see what happens. It’s possible the HVAC module isn’t sending a proper signal. I may pick up one of those pocket scopes so I can check stuff like that. From reading even though the sensor is showing incorrect temp no code is being set because it’s not out of the range the module is looking for which is -60 to +400 degrees F.
I didn't realize when you said 2014 sierra that this is not gmt900
there should be more than just the sun sensor.....
I see they removed the aspirator so it must be somewhere in the duct's or they deleted it which seems odd
all there is now in the headliner is the bluetooth mic right above the driver.
have you check the ambient air sensor reading in the grill?, does the radio show correct outside temperature?
we haven't had any issue's with the 16 or the 18 so I haven't dug into the hvac system on these
 
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mortten

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I didn't realize when you said 2014 sierra that this is not gmt900
there should be more than just the sun sensor.....
I see they removed the aspirator so it must be somewhere in the duct's or they deleted it which seems odd
all there is now in the headliner is the bluetooth mic right above the driver.
have you check the ambient air sensor reading in the grill?, does the radio show correct outside temperature?
we haven't had any issue's with the 16 or the 18 so I haven't dug into the hvac system on these
Ambient scan data shows within 2 degrees of a thermometer and the radio shows correct. I had the headliner dropped to fix an antenna leak. Only other thing up there is the mic. According to the wiring diagram the sunload, ambient light, and inside air wiring all go the the dash sensor.
 

petethepug

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Guys chime in n my hunch here. Didn’t GM update the pickups a year earlier to what our full size SUV are split between 07-14 & 15-20?

Wouldn’t that make his ‘14 Sierra a ‘15 body style?
 

Marky Dissod

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Guys chime in n my hunch here. Didn’t GM update the pickups a year earlier to what our full size SUV are split between 07-14 & 15-20?

Wouldn’t that make his ‘14 Sierra a ‘15 body style?
As in a GMT K2xG?
 

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