P0442 MIL

[wsteele]

It's the front sensors that do most of the work. They are cold started, get a rich fuel mixture and at that point they are not even reading things. They don't read till they heat up to a certain point and then they will feed info to ecm to go from open loop to closed loop. Then the rear sensors have also heated up and they tell the ecm the fuel/air mixture percentage that is coming out of the cats and then the ecm makes adjustments accordingly and then the front sensors deal with whatever fuel mixture ratio there is and again feed info to the ecm. The ecm will make adjustments off of that info and again after input from the rear sensors. I hope I spell this right, the fuel/air mixture the ecm is looking for is 14.7 parts air to one part fuel. That is called
stoichiometric. You want to see the rapid switching with the sensors when you are seeing live stream info. I recently replaced all 4 sensors with GM sensors when my Bank 1 Sensor 2 went bad. Eventually I will do both cats. 135k miles on my rig means some things just need to be replaced to count on the rig.

I think I understood how all that worked, what I was wondering about was your post about the downstream sensors affecting open/closed loop. When I recorded what my Tech 2 was saying about sensor values in post #24, I was watching my engine warm up, it was in closed loop, but the downstream sensors clearly were not working yet.

I was just hoping I might get a little more clarification on when the downstream sensors affected open/closed loop. I know a number of sensors affect open/closed loop, I just haven't read anywhere that indicated that the downstream O2 sensors affected that status or when the engine would go into closed loop.

 

[OR VietVet]

I thought I had said that neither sensor, front or rear, works at all till they warm up and since the front sensor gets the exhaust first it gets hot quicker. As far as me telling you all the steps and what readings to look for and what they should be, all I can say is look it all up in a manual or online. When I ran shops my guys looked for sensor reacting, speed that it reacted/switched, measured temps of cat(s) and wanted to see open to close loop. They also looked at the parameters that they switched in and to see if they were equal and constant and both in the same ranges. You need to remember that I started turning wrenches when it was all still points and condenser and carbed. I became a service manager during that time and stopped wrenching for a living. I then trusted my guys to diagnose correctly and give me correct information to relay to the vehicle owner. I only got more involved with the repair if the vehicle turned in to a problem.
I am old enough now that I would have to do as you. I have to look it all ip. I have an account with ALLDATAdiy.com and I get the same info techs do in their shops for a yearly fee. If I was you, I would sign up for that as well.

 

[wsteele]

I thought I had said that neither sensor, front or rear, works at all till they warm up and since the front sensor gets the exhaust first it gets hot quicker. As far as me telling you all the steps and what readings to look for and what they should be, all I can say is look it all up in a manual or online. When I ran shops my guys looked for sensor reacting, speed that it reacted/switched, measured temps of cat(s) and wanted to see open to close loop. They also looked at the parameters that they switched in and to see if they were equal and constant and both in the same ranges. You need to remember that I started turning wrenches when it was all still points and condenser and carbed. I became a service manager during that time and stopped wrenching for a living. I then trusted my guys to diagnose correctly and give me correct information to relay to the vehicle owner. I only got more involved with the repair if the vehicle turned in to a problem.
I am old enough now that I would have to do as you. I have to look it all ip. I have an account with ALLDATAdiy.com and I get the same info techs do in their shops for a yearly fee. If I was you, I would sign up for that as well.

I must have misunderstood what you were saying. I thought you had said that the cats and downstream sensors affected if the engine would get into closed loop. Everything I had read, and experienced with watching my Tech 2 sensor readings as the engine warmed up in Post 24, indicated the downstream O2 sensor didn't affect closed loop function at all.

I was looking for some clarification on when that might happen as I had not seen it and everything I had read about it said everything downstream of the Sensor 1 didn't affect open/closed loop.

 

[kbuskill]

Just wanted to update this.

I had reset my ethanol content the other day and it stayed at 3% like it should until I filled up the truck today.

I added 26 gallons of ethanol free 89 octane and drove around until the Tech 2 showed that it was no longer learning the ethanol content.

During the process it dropped from 3%, where it started, down to 2% and then 1% and then finally settled back in at 3%.

I think the reason that my ethanol percentage was so far out of whack is that I had ran some E85 several months ago and then went back to my regular Mobil 89 octane.

The problem, I believe, stemmed from the fact that the Mobil station I normally fuel up at isn't very far from my house and I usually fill up on the way home. Since I wasn't driving far enough for the truck to calculate the ethanol percentage before I parked the truck then the truck was forced to relearn the ethanol percentage on a cold start.

I am not 100% certain that my theory is sound but it seems to make since to me.

I will continue to keep an eye on the E% going forward and I will also change my habits and try to start filling up on my way out so it has time to relearn before parking.

 

[wsteele]

Just wanted to update this.

I had reset my ethanol content the other day and it stayed at 3% like it should until I filled up the truck today.

I added 26 gallons of ethanol free 89 octane and drove around until the Tech 2 showed that it was no longer learning the ethanol content.

During the process it dropped from 3%, where it started, down to 2% and then 1% and then finally settled back in at 3%.

I think the reason that my ethanol percentage was so far out of whack is that I had ran some E85 several months ago and then went back to my regular Mobil 89 octane.

The problem, I believe, stemmed from the fact that the Mobil station I normally fuel up at isn't very far from my house and I usually fill up on the way home. Since I wasn't driving far enough for the truck to calculate the ethanol percentage before I parked the truck then the truck was forced to relearn the ethanol percentage on a cold start.

I am not 100% certain that my theory is sound but it seems to make since to me.

I will continue to keep an eye on the E% going forward and I will also change my habits and try to start filling up on my way out so it has time to relearn before parking.

I have probably put enough miles on to know if the E% has moved since I refilled with Ethanol free and reset the value to 3%, but haven’t had time to check the value. I have been out of town visiting my 90+ YO in-laws and doing all the home and car maintenance things he can’t do any longer, phew, what a bunch of work. My last project tomorrow likely will be replacing his air drive rear passenger side door lock actuator, which only seems to close (vacuum side) and not open (pressure side). I will replace it if the Mercedes dealer still carries the part. What a brain dead design for auto door locks.

I am still puzzled why my truck thought it had so much Ethanol as I hadn’t bought E85 in a very long time. I think in my case, there is something wrong in the sensors that have input into the calculation, leading to a longer term issue of pretty rich open loop and then fuel trims getting adjusted back down when it it shifts to closed loop. At least that is what seems to be happening.

 

[wsteele]

OK, got back yesterday and had a chance to check what the Yukon thinks it has in the tank. I am at about a half tank (of the fresh non-Ethanol Mobil I filled it with) when I reset the E level with my Tech 2.

It reads 3.12% Ethanol (essentially the same as it read when I reset it. So far, so good.

I will refill with non-Ethanol again in the next week and will report back after I have ran though a half tank or so. Maybe that Costco Regular isn't all that great after all...

 

[kbuskill]

OK, got back yesterday and had a chance to check what the Yukon thinks it has in the tank. I am at about a half tank (of the fresh non-Ethanol Mobil I filled it with) when I reset the E level with my Tech 2.

It reads 3.12% Ethanol (essentially the same as it read when I reset it. So far, so good.

I will refill with non-Ethanol again in the next week and will report back after I have ran though a half tank or so. Maybe that Costco Regular isn't all that great after all...

I was beginning to wonder if the local Mobil station was jacking up the ethanol content and not telling anyone... lol

 

[wsteele]

I was beginning to wonder if the local Mobil station was jacking up the ethanol content and not telling anyone... lol

Funny, I was thinking the same thing about my cheapo Costco may be doing the same, or the disti, or the refiner... Truth be told, we only have trust to go on.

Although the open loop idle is pretty smelly when it starts getting up there in reported percentage.

 

[kbuskill]

So I filled up with my normal Mobil 89 tonight.

The truck took 26 gallons so I would expect, with the not more than 10% ethanol sign on the pump, that my ethanol percentage would have been somewhere under 10%.... but no.

After the ethanol percentage fluctuated all over the place, from 5% up to 30%, it finally settled in at 18.5% according to my Torque Pro app.

I don't know what to think at this point. Either my truck is not calculating it correctly or the Mobil station is selling me more ethanol content then they are claiming.

Maybe next fill up I will try a different station and see what the results are.

 

[wsteele]

So I filled up with my normal Mobil 89 tonight.

The truck took 26 gallons so I would expect, with the not more than 10% ethanol sign on the pump, that my ethanol percentage would have been somewhere under 10%.... but no.

After the ethanol percentage fluctuated all over the place, from 5% up to 30%, it finally settled in at 18.5% according to my Torque Pro app.

I don't know what to think at this point. Either my truck is not calculating it correctly or the Mobil station is selling me more ethanol content then they are claiming.

Maybe next fill up I will try a different station and see what the results are.

I haven’t had the opportunity to refill yet. My current % has not moved off 3% since I reset it and have been running the zero ethanol pump gas.

I think I may try my usual Costco “not more than 10%” and see if it pops way back up where it was when I discovered the high 30’s % ethanol content. If it does pop back up. I will reset it again and verify that it jumps back up again. Then I will run it down to near zero and refill with the zero ethanol pump gas. This time, I may hold off resetting amd see if the truck tracks back down percentage wise to 3%.

It is very confusing, and I suspect part of the problem rests somewhere in the GM flex fuel software. But I am definitely also thinking the gas companies may be spiking their gas with higher ethanol content, perhaps on a random basis.

I think if we run down enough blind alleys, maybe a pattern will emerge and some conclusions drawn.

PS - I think I said the zero ethanol gas I was using was Mobil, that was a brain fart as the zero ethanol pump gas I have been using is 76.