You're within spec, but as I re-read your original post, what OS did that vehicle come from? We may be using the wrong diagnostics for the code if that OS doesn't match the vehicle.
Also, you're on the low end of what is normally seen for amperages. I'm wondering if it is dipping out of bounds on the low side, but lets investigate the OS thing first.

 

It is the 12587603 is now. The engine was from a 2004 suburban 1500 flex fuel (L59) and I’m about 80% sure the original operating system was 12579405.

 

I think we have the answer now. That operating system is used in a few vehicles as the stock OS. I looked up one of them, a 2004 Astro, and the max amperage spec is only 1.3 amps. You're measurements put it right up against that number. The operating strategy is different too. When the O2 sensor is turned on it is briefly pulsed off for 50mS of every second. When turned of it is pulsed on for 50mS of every second to test the circuit. It is also only turned on if the engine is running, like you noticed. My bet is that you're going to have to either change the OS or get an O2 that operates in that heater range. Just for giggles, do you happen to have a 1 ohm sensor hanging around? I wouldn't spend money to buy one, but if it was hanging around I would try wiring it in series with one of the heaters and see if the code disappears on that bank.
Joe

 

That’s very interesting and I would have never thought to look at the operating system.

As a matter of fact, I do have a pack of resistors and I’m sure there’s a 1 ohm in there. Can I just wire that into the 12v supply wire up at the fuse box? And if that works, is that a mod that can stay there?

 

This would be uncharted territory, so I'm not sure if it'll even work. Make sure the resistor is rated for the wattage you're running.
Even if we get the amperage in range it may reduce the effectiveness of the heater to where it takes too long to get hot. Maybe to test it you install the resistor in place of the fuse. Take a blown fuse and solder the resistor across the back of it, then pop the fuse back in place. Clear the codes and measure the amperage again, If it's better, take it for a ride.
I'm pretty cheap, so I'd definitely try fooling with it before I replaced the O2 sensors. Plus, it's free and easy to try.

 

I have 0 ohm and 10 ohm resistors. No 1 ohm resistors.

I’m using PCM hammer to remove unnecessary diagnostic codes. Is there a setting or another way I could just change the current range the pcm is expecting?

It will be raining here for a while so I probably won’t make much progress on it until it clears up maybe Saturday or so

 

Maybe start wiring the 10 ohm resistors in parallel until it gets you an amperage that doesn't set the code.
Also, if you just want to kill those 2 codes and see what happens, give it a try. Make sure that with the codes turned off that the PCM still turns the heaters on. If so, you may be good to go unless you set codes for them taking too long to wake up.
Both things are free to try, and would be an interesting experiment.
Joe

 

That not a bad idea. So if I turn off p0135 and P0155 the will the pcm still even use the o2 sensors to adjust air and fuel, or just ignore them? If it does I’ll just do that and check the heater resistance every oil change or so.

 

I'm not sure. Give it a try. See if the fuel trims still bounce around while you drive it.

 

Alright I reflashed my PCM and removed both P0135 and P0155. I also replaced an old ground strap from cylinder head to body. And added a 2ga ground cable from the engine block to the frame (not present previously). Connections were wire wheeled, cleaned with alcohol, connected tightly, and have dielectric grease. All grounds measure out to <0.5 ohms.

Cold startup behavior is smoother, the throttle response feels slightly crisper and my neighbor even commented on it sounding smoother.

P0135/P0155 codes are gone (removed them in the pcm) and I can still see both long term and short term fuel trims bouncing around while driving. I have not had a chance to measure low side o2 heater current at pcm yet though.

Is there any way to tell if the pcm considers or ignores the o2 sensor readings? Is that a closed/open loop thing?

 

If your fuel trims are active then the pcm is watching and using the O2 sensors. There's no other data that makes them move. That may be all you have to do to fix this. It may even be the best fix for it, especially because both the heaters are still being cycled. Using resistors to bring them into spec will only cool them down, and there's no worry about running too many amps through the pcm drivers. They can easily handle that and more. The only down side is when you eventually do have a heater failure you'll have to spit it without the code being set.
Joe

 

Sounds good! I appreciate all your help on this Joe, I learned a lot.