Check Engine Light - No Codes

I have an oldie with an ECM problem, an 83 Pontiac STE 6000 with a carb'd 2.8 that has an ECM with no codes. The ECM will only come on with a warm engine and will turn off if I restart. I 've cleaned the
ECM ground. The O2 sensor will have cross-counts and range from .3 to . 8V and then stick at .8V at idle. The voltage will drop to .3V if I rev the engine. The O2 sensor was replaced 5K miles/5yrs ago. No bad O2 codes. Do I have a bad O2 sensor. Other ideas?
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What do you mean the ecm will only come on warm.?

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You've done pretty good with an O2 sensor lasting that long. They are very sensitive and once dirty don't send the proper messages. I've also seen them with bad wires.
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Sorry for the late replay all. Retesting I did track down the Check Engine Light problem to a stuck TPS at full travel, but would only come on until about 30 minutes of running and throw a code 21 for TPS There is a 180 deg lower temp thermostat in the car, which may cause the long time to throw the code by staying Open Loop. Standard is 195 deg, although 180 was listed as alternate. " ECM come on warm" - What I meant to say was the CEL will only come on when the engine is warm to operating temp after 30 minutes. I need to retest the O2 sensor since I was probably running rich with the TPS at full travel.
With the TPS fixed there is now a rough idle and some hesitation. Maybe this indicates a lean condition since the TPS enriched the mixture. Maybe the lean out is caused by vacuum leak? Any other Ideas?
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