Nate writes:

Ok, you asked for input and I’ve got a question about my 2003 Cadillac CTS. I figure I’m more likely to get a reliable answer from you and the best & brightest of TTAC than the goof balls at Car Talk (this letter is from February-SM), so I’ll ask.

I bought this CTS back in November. It had 135,000 miles on the odo, came from a private owner and apparently had significant engine work accomplished a year or so ago apparently as a result of a timing belt failure after it wasn’t replaced on schedule. Before being able to get the car licensed, I paid to have the thermostat and temp sensor replaced as I had a CEL and a P0128 code and the car wouldn’t pass inspection with a CEL code. The code came back after just about 1 week.

The CEL will clear if the ambient temps move up above 45-50 degrees but returns when the temps get back down to Utah normals for winter. I’ve been unable to find an online solution. I’ve considered installing a temporary partial radiator block, (cardboard & duct tape) to see if that old school fix brings the temp up. The car doesn’t have a temp gauge- thanks for nothing GM; but seems to warm up the cabin appropriately if not exceptionally fast.

Am I going to have to reset the codes each December before taking this in for emissions inspection or is there a real fix?

Sajeev answers:

Much like LSX swaps for people wanting to make a slow car fast, much like Panther Love for someone wanting a cheap and durable ride, I pretty much always think Dex-Cool is the problem when certain vintage GM products have temperature control problems.  As this paragraph shows, Dex-Cool is not my friend…and I am somewhat less goofy than the Car Talk peeps.

On the plus side, others are in your situation and they agree with me. Let’s face it: the timing belt proves that this car was neglected.  It’s a safe bet that Dex-Cool was never changed either, possibly topped off with non Dex-Cool compatible fluid too.  So there is a TON of the stuff you see in the photo below. And above. So I suspect that the 1 week grace period you mentioned was the time necessary to re-clog that temperature sensor and cause the P0128 to trip yet again.

I’d recommend a closer look at your cooling system, probably replacing the radiator too.   Just be careful how aggressively you remove Dex-Cool from the cooling system, you could flush it all out and get a ton of Dex-Cool “snot” stuck in the heater core. Which means you no longer have a heater. Which means…well, have fun removing the interior to get the heater core out. In a Utah winter. Damn, Son…

Sorry, I wish I saw another way out.  Maybe the B&B can help.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry.