Heated O2 Sensor Monitor

[94 Jimmy]

Thought I'd try here, since the vehicles are similar.
2000 Yukon, 6.0L, JBA Headers and exhaust, 70,000 Mi.
I had to clear a P1870 code from the ECM, this has the unfortunate side effect of clearing all of the Diagnostic Monitors. I've been through the suggested GM Drive Cycle several times but can't get the Heated O2 Sensor and Catalyst monitors to flip to complete. My OBDII reader shows a normal upstream O2 sensor graph and the downstream O2 sensor shows a nice lean exhaust. I replaced the upstream sensors just in case, but no luck. I haven't set any codes referring to the o2 sensor or cat, but the monitors just won't complete.
What is the computer looking for to complete the Heated O2 Sensor monitor. There is no circuitry in the heater to monitor current or voltage to the heaters, just a direct connection to the fuse. Does the computer look at the time for the sensors to go to closed loop, if so what is the time. My Olds minivan will go to closed loop in 1-2 minutes, but the Yukon takes about 4 minutes. If I leave the key on the sensors get warm, so the heaters are working, but is the steel tube exhaust conducting the heat away faster than the stock cast iron headers and slowing the time to closed loop? Could I heat the area around the sensors with a torch before I do a cold start and hurry the transition time?
I need to get these completed so I can SMOG this thing and get to work on the transmission.
Any ideas? Thanks.

[maxwedge]

Until the heated o2 sensor comes online the cat monitor will not be ready as one affects the other. How much have you driven the truck, started cold ,done the whole routine?

[94 Jimmy]

I've been through the cold start sequence at least 10 times, probably more. It takes about 4 minutes for the sensors to switch to Closed Loop which they do at the same time so both sides are balanced. The upstream sensors are new and are acting as they should, the down stream sensors are the old(70,000 mi) units, but start rich (.8V) and move to lean (.2V) when they go closed loop.
I have steel tube header and exhaust on the truck, is it possible that the exhaust flow is so spread out and the exhaust tubes such a good heat conductors compared to cast iron that it takes longer (too long) for the sensors to heat up and go closed loop.
This weekend I think I may take a torch and heat the area around the sensors to see if they go closed loop faster. My Olds minivan will get to closed loop in a little over a minute. This is the reason for the question about what the computer wants to complete the sensor monitor. Is it time or some other characteristic.
I'll let you all know what I find, I think I'll call JBA Exhaust tomorrow and see what they say about their exhaust.

[maxwedge]

Are you sure the heated sensors are monitored, it may show on the scanner but may not be part of the program in 2000, enhanced obd11 was, check into this. I know my 2002 TB is.