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Mike S.
Mike S., ASE Certified Technician
Category: Cadillac
Satisfied Customers: 7568
Experience:  ASE certified
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I HAVE A 2003 CADILLAC CTS AND NO INTERIOR HEAT. I have been educating myself about this and still have questions, and still no heat. its going to be 14 o tonight. So far I checked my antifreeze and its good, my obd2 showed no codes, I have not checked fuses yet as I have air, but, it's just cold. my engine is not running hot and the hoses going in to the interior are warmish, not hot, one warmer than the other. it blows strong, seat heaters are at least keeping my backside toastie. Lets see, it is the same way ,cold, on both driver and passenger sides. I have read that there is an actuator somewhere in the dash, but, don't want to DIY there. A mechanic told me it might be the thermostat which would be an easy fix, but, the engine seems fine. Maybe a little loss of pressure when driving, sort of like a loss in compression. If ya'll can give me an idea of a way to do a diagnostic on this it would be great and worth every penny. It's cold out there. [email protected]

Chances are if both heater hoses are not hot then either the heater core is clogged and not allowing coolant to flow through it or the Coolant Bypass Valve is not working allowing coolant existing the heater core to return to the radiator and not the engine block, preventing it from reaching operating temperature and thus heater performance may be affected.

Engine vacuum from the by-pass solenoid controls the coolant bypass valve positions.

For more info on that Coolant Bypass Valve including replacement, click here. The Coolant Bypass Valve is operated from vacuum applied by the coolant bypass solenoid, so check the vacuum hoses to and from both for being off or any holes or leaks and the electrical connection at the solenoid as well as the fuses.

Customer: replied 6 years ago.
I could not print or open this doc on my print hooked up desktop, but looked on laptop and am impressed with your answer. Can you send "click here" pages in a different format or e-mail them or something that might work ? [email protected]
I want to get right on it to see if you are right and to warm my ___
Get a hair cut, bro. -

just kidding, I wish I had some.
I have had alot of people lately saying they couldn't open *.doc's that I uploaded. The first question I have to ask you is do you have microsoft's office or word installed? That is a doc file that opens with word if you don't have office or word installed, microsoft does have a free word viewer here,
Mike S. and 3 other Cadillac Specialists are ready to help you
Customer: replied 6 years ago.
Dude, you rule. I am in Mechanics 101 and my thesus is a 1952 chevy 5 window rat rod. I'm having a blast and amybe someday I'll handle some really scary auto project. I was about to tear up my dash looking for an actuator and you stopped me thank god. Well done. Is that a graduation photo from the 80s or what ?
Yeah, 1977.
Customer: replied 6 years ago.
your input has been approved. I am a 1970 grad
Customer: replied 6 years ago.
could the part , coolant bypass valve also be called a heat sensor. it looks like a large black plastic unit with a smaller one attached by small "hose"which might be the solenoid. I can clone and send pic if that would help. Sorry to re ask but the cadillac part guy didn't readily know what the part was either. He found it at $140 but the one I mentioned is $80 on ebay. I like that price better plus dealership are always way over priced. Ebay calls it a heater sensor though.
Chuck at XXX-XXX-XXXX [email protected]

Rockauto lists the Heater Bypass Valve for $70.79

2003 CADILLAC CTS 3.2L 197cid V6 FI (N) DOHC : Heat & Air Conditioning : Heater Bypass Valve Wiki Price
ACDELCO Part #(NNN) NNN-NNNN{#25740115}

* Non-stock item--shipping delayed up to 5 business days
Add to Cart


Edited by Cadillac_Man on 12/15/2010 at 6:47 PM EST

Here it is for $67.08

Acdelco 15-31971
Heater Water Bypass Valve
Picture Brand Part Number Alternate Part Number Description On Hand Core Price Sell Price Add to Cart
15-31971 25740115 Heater Water Bypass Valve
In Stock $0.00 $67.08

Here for $63.53

CATEGORY: Hose (shipped only to local fedex hub 4 pickup)
GM LIST: $124.56
OUR PRICE: $63.53


Customer: replied 6 years ago.
excellant. Rockauto has it and its on the truck. Dude give me your contacts and we can do more business.
chuck(NNN) NNN-NNNN [email protected]

You know the GM place wanted $218 because I'm not from NC, weird that they would do that.
You can use my profile to contact me in the future,