: Mystery Check Engine Light

05-24-14, 10:51 PM
I have a 2000 DHS, all options (even night vision) that I purchased used from a friend. Car has 51K miles. The problem from since it was new (and the Dealer could not correct) is the check engine light comes on intermittently and the only code is P0154.
Itís (sensor) been replaced. Now here is the strange part. When the outside temp gets above 50 degrees, it clears, so the light stays off all summer. Late in fall when itís cold, it shows fault and the light comes on and stays on most all winter. Occasionally, in the winter) the light will go off and then come back on soon after. Neither mileage nor performance suffers when the light is on and it shows the error code. No other problems other then new battery after 12 years and new water pump last year. Very little oil consumption between oil changes. This has been driving me, and the previous owner nuts for the last 14 years. Anybody have any ideas? (sorry for the long winded question)

05-25-14, 10:14 AM
Knotty problem - that an O2 sensor would function differently due to ambient temperatures.

Just for shits and grins, closely compare the DIC indicated ambient air temp display with bank and business temperature signs for a while - or even stick an el cheapo Taylor thermometer out the window for a while while driving. IF your ambient temp display seems to be incorrect, a new sensor (on the hood latch/radiator brace) is about $12 at a NAPA store. Might be worth a try - ambient and coolant temps cause certain emissions controls to change modes.

BTW, Welcome to the Looney Bin !!!

05-25-14, 10:48 AM
I agree. Ambient air temperature should have no effect on an O2 sensor.

05-26-14, 09:25 AM
No, the Temp reads within a degree or two of the various time&temp/radio readings. With 50 to 60 degrees being the temp range for the issue to either occur or clear (depending on getting warmer or colder for the year) Air density difference is non existent, The O2 sensor itself has no way to know ambient air temp. A faulting MAF would show up on stored codes as well as the car stop running correctly (if at all) I'm glad the beast has it's own little personality quirks, but this is crazy.

05-26-14, 03:28 PM
As a Follow up. The prior owner related the Dealer's final solution. "Keep driving it until something breaks and have it brought in, or you can spend $700.00 and still not know what the problem is." Fine while it was still under warranty, but considering it was nearly a $60K car when new, and the problem was from day 1... well, if this Sate had a Lemon Law at the time, I would have told Mr. Cadillac to build me another special order just like this one and you can have this one back, thank you very much. Good to know the best diagnostic tool the Dealer had was smoke curling up out of the car as it's being hauled in, That they could figure out. :-) !
Hopefully some one has experienced this before or has a clue or two on how to proceed. The car IS perfect in every other way, it's still an awesome vehicle, it just enjoys mocking me with it's mystery trouble light.

05-26-14, 03:42 PM
:noidea: I'm out of ideas.

05-27-14, 10:46 AM
Where does your temp gauge run under most conditions ?

The emissions controls change loop operation between 135 - 160 degrees coolant temp, depending on ambient temps. You may have a loop change data hiccup.

05-27-14, 11:45 AM
Temp stays right at 12 o'clock straight up winter or summer. It may rarely go to a smidge to the right of 12 o'clock if it sits idling for an extended time, but never out of the solid green bar on the diagram. In the most severe cold, it starts right up and the temp raises smoothly to the green bar range. The temp gauge moves so little with the action of the thermostat, you have to be staring at it to notice the movement
Now this is an interesting possibility. Thanks!!!! Think I'll look for that in the Service Manuals (prior owner ordered them. Very scary reading. Makes me yearn for my '63, 352 Ford Galaxy. A screw driver, crescent wrench and a ball peen hammer was pretty much all you needed for major repair. :-) ! )

05-27-14, 11:54 AM
Same for my old '65 Chevelle Malibu SS 327/350 or the '67 Cougar 289 HiPo. Wish I still had them both.

Your temp gauge operation exactly mirrors mine.

Here's another possibility - CKP hiccups. One symptom of a failing/sluggish CKP is a slow speed stall right at loop change - and that sets a SES light. The originals ("AC Delco") were made by Siemens in Mexico and had a high problem rate. The replacements (AC Delco) are Denso from Japan.

Wanna spend $170 ? Take a look at a Scangauge-II. www.scangauge.com (http://www.scangauge.com) . I monitor coolant temp, intake air temp, ignition advance and MAP (inverse manifold vacuum), but you can monitor any 4 of 12 installed gauges as well as create 8 more of your own (if the OBD-II data stream supports them) - and now you can daisy chain 2 modules together in order to watch 8 gauges at once. Plus, it's a scan tool and will turn off the SES (until the fault occurs again).

05-27-14, 02:29 PM
Had an 02 Deville that would get a SES light at 50 - 52 degrees or below, usually P0101 but I did see others. Turned out to be a MAF, bought a reman'd Cardone from O'rilleys that did the trick. The performance or mileage never suffered but you could clear the code and it might not set again that day, but there is one bridge that always had cold gusty winds blowing across it and everyday going home the SES light would come on as I crossed it.

05-27-14, 06:45 PM
As I was told, the crank position sensors were replaced almost immediately after delivery with the new type. (an oops moment for GM) BUT, perhaps a contributing factor. Which begs the question, how long are those sensors good for? The power application is smooth and the sensor will fault at any time.. from idling to driving at 55 (another obstacle in pinning down the cause.)
we're getting closer!! Thanks guys!
(the scan tool sounds awesome)

05-27-14, 08:59 PM
how long are those sensors good for?
Generally speaking (and with the obvious exception of the faulty 2000-2003 CKP sensors from Siemens) they are pretty reliable and rarely ever fail.

05-28-14, 03:35 AM

05-28-14, 01:27 PM
oh, yes. that list was the first thing consulted. and of all the causes possible, none really fit the situation. Now the car was due for it's emissions test in March and since the weather was cold, the ses light was on. so I procrastinated until it warmed up in may (they give you until June.) and while I was on my way to the grocery store, the light went out, Made a quick detour to the testing station and it passed with a perfect score. The heated O2 sensor was reporting "ready" and the only stored code was P0154, the same as the car gave me when I queried it. As I said before, the car runs just fine with the fault showing or not. it just makes no sense. in years past, the guys who would troubleshoot auto electrical systems were part mechanic and part mind reader. With todays multi input computer controls, good luck to us all. If I had a big enough garage, I'd buy a 2000 junker as a test bed and start tearing into it trying to find a potential flaw. (everyone needs a challenging hobby :-) !)

05-30-14, 03:28 AM
Well, we are now firmly in summer time mode and nary a blink from the SES light. Thanks for all the input. It has given me great info. I now await the Fall and cold weather with Scan tool in hand ! :-)
(to be continued)