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Discussion Starter #1 (Edited by Moderator)
The check engine light has been on for a couple of months on a 2008 Acadia SLT with 62,000 miles. OnStar hasn't been sending monthly diagnostics, so I called them and they said the DTC was P0449; most likely a fuel cap leak. I had the same thing two years ago with a 2001 Seville, got robbed at a smog check station for a new gas cap in order to pass emissions. So I bought a new gas cap, code still set. Problems with my other car caused me to buy an OBDII reader and after I got that car to pass smog.

I tried the OBDII reader on the Acadia. Only code stored was P0449. Searching archives it seemed the next most common failures were the purge valve next to the throttle and the vent valve on top of the gas tank. From the discussions it seemed the vent valve on the fuel tank seemed to set the P0449 code, while the purge valve would normally set other DTCs, not just P0449.

However I had a full tank of gas and it would be much easier to check/replace the purge valve. Only a couple of minutes to get it out and do a manual vacuum check. It seemed one direction it was slowly losing vacuum, but it was not leaking when air was blown into the the purge valve on either side. The test seemed inconclusive so I reinstalled the old valve. started the engine to make sure the car would run, and the check engine light was NOW off, and has stayed off thru three different restarts. Maybe reseating the connectors on the valve was all that was needed.

I have a biennial smog check in three months so I ordered a Delco replacement from Home Depot. Not only was it $20 cheaper than at some major online auto parts stores, but there was no shipping charge if I pick it up 2 miles from my house at the closest Home Depot. I couldn't find the Delco part in stock locally, so I would be waiting anyway for a shipment from the other places. It's just strange ordering auto parts from Home Depot. Since the check engine light is now off, the problem is fixed until the part arrives and I can install it then.

(mod edit - separated post into paragraphs to make it more readable)
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