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A couple of months ago I got a P0420 code. I took it to the shop and being that the car only has 50K miles we deceided it was a faulty HO2 Sensor (after cat). The problem went away for a couple days then I got a P0141 (P0420 never came back so I'm thinking the cat is o.k.) I took the car back to the shop and the tech deceided the new sensor must be no good.
He replaced the sensor and a couple days later I got the P0141 code again. It seems the sensor is failing the cold start test(The heater calculated resistance is not within an expected range).
According to my manual the heater resistance test is done at startup when
DTC's p0030, p0036, and p00050 are not set (they're not)
The ignition if off for more then 8 hours(it was)
The ECT sensor is between -22f and 113f (It was)
The difference between the ECT and IAT is less then 14.4F
The last 2 times the code was thrown those were the conditions
Does anyone have any idea's on what the problem might be
my thoughts are its either a wiring problem to the PCM(connector 2 pin 56), a bad PCM, or aftermarket sensor is not sufficiant.
I may have to bring it to the dealer if I can't figure it out
I dont think the cat is bad because P0420 never came back.

Took it to the dealer turned out to be a bad PCM they replaced for free which is odd because I have no warrenty left
but they said it was covered and I didn't want to ask too many questions

That's great to hear. Also, a bit surprised as to what the cause of the problem was.

I was gonna ask what brand sensor you used, if it comes back that could be part of the problem.

Well it came back tonight. I'm gonna have to bring it back again. I don't know what brand sensor it is. I had it installed at my friends shop.

I'd get an AC Delco sensor installed. I have a feeling it's still the sensor. Take a good look at the wiring to the sensor too.

PCM replacement is usually the last step in the troubleshooting. The reason why it was replaced for free was because the PCM is the key component in the entire emissions system and all manufacturer's under a Federal mandate, must warranty repair any emissions related failure/component for either 7yr/70k or 8yr/80k (It's been awhile, I can't recall, it may be slightly different now).

As to an O2 sensor, on a GM vehicle, ONLY replace it with GM or AC-Delco HO2S. I know it may sound absurdly stupid, but GM PCM’s can be VERY finicky when running random I/M system tests and readiness monitor checks on the O2 sensor (just as in your case of checking the resistance in the circuit while heating), as well as normal monitoring of operating fluctuations.

I’ve always like Bosch sensors personally, but GM designed certain tolerance thresholds into their PCM’s and it requires usage of a controllable set of equipment parameters they’ve built into that system. As such, the GM/AC-Delco HO2S sensors are key in this circumstance. I speak from experience.

Well sorry it took so long to get back to this but here it is. The new AC Delco sensor was installed about 2 weeks later light came back but this time the code was p0420 cat oout of range. Which was the original code when all this started. Brought it back to the dealer and they said it was a bad cat. They replaced it for free along with the exhaust pipe and both mufflers all free. I guess it was covered under some kind of federal emmisions law.

Yup, the federal emissions warranty is 8 years 80k or something like that, and since the DP back is all one piece, that's the GM replacement part for it.