Heater will not turn on if the lights are on

PSJ011777

MEMBER
2005 PONTIAC GRAND PRIX
3.8L
6 CYL
TURBO
2WD
AUTOMATIC
145,000 MILES

Very weird problem. Car has the auto sensor lights, if the lights are on because of the sensor the heater will not turn on, and the heater will not turn on if the sunroof is open either. Last thing I noticed is the heater will not come on unless the water temperature is up to 200. Battery cables have been checked and did not find any problem. Heat instrument cluster has been pulled out and checked all connections. Fuse has been checked. I have plugged in my scanner and it shows no codes at all.

3 Replies

JIS001

MECHANIC
3,411 POSTS

What type of scanner are you using and which control modules are you looking at for codes? Can you look at parameters with your scan tool also? And can you clarify on no heater? Does the fan come on but no hot air is coming out?

Do you have the electronic climate control system? Do you also have duel or single temp control? Also, do all the other functions work properly?

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Sunday, November 19th, 2017 AT 2:33 PM

PSJ011777

MEMBER
34 POSTS

The scanner is just a simple Centech scanner just to read the ABS and OBDII codes, clear them when fixed and can display live sensor data. The blower does not kick on at all if the automatic lights turn on. Single climate, and just the normal 3 switch climate control. If the automatic lights are on it doesn't matter if the temp is in hot or cold the blower motor will not kick on. The radio, and everything else work fine.

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Monday, November 20th, 2017 AT 11:44 PM

KEN L

MASTER CERTIFIED MECHANIC
47,643 POSTS

To me this sounds like a bad ground for the BCM or the BCM is going out. Here is a BCM wiring diagram (below) But to make sure its not one of the 5 fuses that control the BCM we should test those first.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Here is the fuse locations as well (below)

Please let us know what happens.

Cheers, Ken

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