P0302
Moderator
Joined: 01-23-2009
Posts: 26,537
From: Fredericksburg,VA
Actually, you got it backwards. P0171 and P0172 are TRIM system rich or lean. Meaning the "AF system" is the opposite.
What you are reading as "lean condition" is really "TRIM condition is lean because the O2 sensor thinks the there is too much fuel in the AF without correction".
What you are reading as "lean condition" is really "TRIM condition is lean because the O2 sensor thinks the there is too much fuel in the AF without correction".
Senior Member
Joined: 12-05-2014
Posts: 3,370
From: Northern Ohio
Actually, you got it backwards. P0171 and P0172 are TRIM system rich or lean. Meaning the "AF system" is the opposite.
What you are reading as "lean condition" is really "TRIM condition is lean because the O2 sensor thinks the there is too much fuel in the AF without correction".
What you are reading as "lean condition" is really "TRIM condition is lean because the O2 sensor thinks the there is too much fuel in the AF without correction".
I had a P0172. It was blowing gray smoke at startup, maybe even needed flood clearing pedal to the floor to start, obviously rich, ran great once it quickly got to “closed loop.”
P0172 is describe as “Fuel Trim System Rich.” The stored ethanol value was incorrectly high, causing the system to start too rich, then it managed to correct itself once in closed loop, trimming the system leaner, but signaled that it thought the needed correction of the rich condition was excessive, ergo, P0172.
All seems consistent to me, if you interpret P0172 to mean something to the effect of, “required fuel trim excessive, system was too rich.”
Last edited by PulpFriction; Jan 18, 2023 at 10:00 PM.
New Member
Joined: 12-05-2022
Posts: 12
From: Barnhart, Missouri
I was think the same thing but decided maybe it is a "Chicken and Egg" thing... what came first?
System sensed Lean conditions then ECM dumped fuel to create a Rich Condition... setting the p0172
OR
System registered a Rich condition and reduced its trim to lamda... sets the p0172
I am confused just typing this...
System sensed Lean conditions then ECM dumped fuel to create a Rich Condition... setting the p0172
OR
System registered a Rich condition and reduced its trim to lamda... sets the p0172
I am confused just typing this...
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Joined: 05-01-2014
Posts: 8,512
From: California
Senior Member
Joined: 12-05-2014
Posts: 3,370
From: Northern Ohio
Yeah, er, no, er sort of, also, kinda the opposite. If the car thinks it has E85 and it doesn’t, it will be very rich, at startup, until it can correct itself. Read on.
Some of what I know about this is from Donbrew, some personal experience, some from the font of all wisdom, the interweb.
First, what is “E85”? Lotsa people think it’s 85% ethanol. Once upon a time, maybe, but not any more. It’s anywhere from 51-83%. And the gas station feels no obligation to tell you how much, and would you believe them, anyway? In any event, a FlexFuel is spoda accept whatever ethanol you pump, from 0-85% ethanol, willy-nilly, no need to be consistent.
So the car needs a way to figure out what % is in the feedbag. Nope, no “ethanol sensor,” per se, in an HHR FlexFuel. Some fuel system honcho at GM decided we don’t need no stinkin’ sensor. We have O2 sensors.
When she’s running, the O2 sensors provide the nag’s brain the signal it needs to send the injectors the correct amount of fuel to get O2 level to a happy place. Now that brain, knowing how much fuel and how much air (thank you, MAF sensor,) knows what A/F made fragrant exhaust. Since a eutectic gasoline burn is 14.something/1 and ethanol is 12.something/1, brain can calculate the ethanol % in the feedbag, right? Right, provided are all fit and fine. Some when all goes well, the brain can remember the %, and send suitable shots of fuel through the injectors at startup, when “open loop” is needed because the O2 sensors are still cold and senseless. The car within a couple minutes warms the O2 sensors enough to go into “closed loop,” and now the brain has the feedback it needs feed fuel for exhaust excellence. This as we know is called “fuel trim.”
Apparently, adding about 3 gallons or more of fuel prompts the car to do all this figuring. It’s been claimed that the ignition must be off, too, but I haven’t been able to verify that, but no matter, you should always shut down when fueling anyway, you reckless scofflaws.
So to sum up (thank you for your patience,) if the computer thinks there is E85 in the tank and there isn’t, it thinks it needs more fuel than it does, and will (try to) start very rich. If it succeeds without stalling until it’s warm enough to correct the rich condition with fuel trim, it will recognize that the trim needed is excessive, and set a P0172, “Fuel Trim System Rich.”
So, “WHY would the system think it has E85 when it doesn’t?” Well, if GM know, they aren’t saying. But to sense the % correctly, everything needs to be working, “the MAF sensor, injectors, plugs, catalytic converters, O2 sensors and hoo nose what else,” remember? But they know it happens, albeit rarely, and I’ve found several instances in addition to mine, one of which was a (2012?) Silverado, AND there’s a TSB on it. It doesn’t say much except, reset the % to zero. A workaround that has seen some success is to pump the car full of E85 so it won’t choke on startup.
When the system stores a falsely high %, how hard it is to start depends upon how far off it is. Also, it seems the system at least sometimes gradually corrects itself.
This by no means should be taken to mean this situation is the only reason for a P0172.
Some of what I know about this is from Donbrew, some personal experience, some from the font of all wisdom, the interweb.
First, what is “E85”? Lotsa people think it’s 85% ethanol. Once upon a time, maybe, but not any more. It’s anywhere from 51-83%. And the gas station feels no obligation to tell you how much, and would you believe them, anyway? In any event, a FlexFuel is spoda accept whatever ethanol you pump, from 0-85% ethanol, willy-nilly, no need to be consistent.
So the car needs a way to figure out what % is in the feedbag. Nope, no “ethanol sensor,” per se, in an HHR FlexFuel. Some fuel system honcho at GM decided we don’t need no stinkin’ sensor. We have O2 sensors.
When she’s running, the O2 sensors provide the nag’s brain the signal it needs to send the injectors the correct amount of fuel to get O2 level to a happy place. Now that brain, knowing how much fuel and how much air (thank you, MAF sensor,) knows what A/F made fragrant exhaust. Since a eutectic gasoline burn is 14.something/1 and ethanol is 12.something/1, brain can calculate the ethanol % in the feedbag, right? Right, provided are all fit and fine. Some when all goes well, the brain can remember the %, and send suitable shots of fuel through the injectors at startup, when “open loop” is needed because the O2 sensors are still cold and senseless. The car within a couple minutes warms the O2 sensors enough to go into “closed loop,” and now the brain has the feedback it needs feed fuel for exhaust excellence. This as we know is called “fuel trim.”
Apparently, adding about 3 gallons or more of fuel prompts the car to do all this figuring. It’s been claimed that the ignition must be off, too, but I haven’t been able to verify that, but no matter, you should always shut down when fueling anyway, you reckless scofflaws.
So to sum up (thank you for your patience,) if the computer thinks there is E85 in the tank and there isn’t, it thinks it needs more fuel than it does, and will (try to) start very rich. If it succeeds without stalling until it’s warm enough to correct the rich condition with fuel trim, it will recognize that the trim needed is excessive, and set a P0172, “Fuel Trim System Rich.”
So, “WHY would the system think it has E85 when it doesn’t?” Well, if GM know, they aren’t saying. But to sense the % correctly, everything needs to be working, “the MAF sensor, injectors, plugs, catalytic converters, O2 sensors and hoo nose what else,” remember? But they know it happens, albeit rarely, and I’ve found several instances in addition to mine, one of which was a (2012?) Silverado, AND there’s a TSB on it. It doesn’t say much except, reset the % to zero. A workaround that has seen some success is to pump the car full of E85 so it won’t choke on startup.
When the system stores a falsely high %, how hard it is to start depends upon how far off it is. Also, it seems the system at least sometimes gradually corrects itself.
This by no means should be taken to mean this situation is the only reason for a P0172.
New Member
Joined: 12-05-2022
Posts: 12
From: Barnhart, Missouri
Yeah, er, no, er sort of, also, kinda the opposite. If the car thinks it has E85 and it doesn’t, it will be very rich, at startup, until it can correct itself. Read on.
Some of what I know about this is from Donbrew, some personal experience, some from the font of all wisdom, the interweb.
First, what is “E85”? Lotsa people think it’s 85% ethanol. Once upon a time, maybe, but not any more. It’s anywhere from 51-83%. And the gas station feels no obligation to tell you how much, and would you believe them, anyway? In any event, a FlexFuel is spoda accept whatever ethanol you pump, from 0-85% ethanol, willy-nilly, no need to be consistent.
So the car needs a way to figure out what % is in the feedbag. Nope, no “ethanol sensor,” per se, in an HHR FlexFuel. Some fuel system honcho at GM decided we don’t need no stinkin’ sensor. We have O2 sensors.
When she’s running, the O2 sensors provide the nag’s brain the signal it needs to send the injectors the correct amount of fuel to get O2 level to a happy place. Now that brain, knowing how much fuel and how much air (thank you, MAF sensor,) knows what A/F made fragrant exhaust. Since a eutectic gasoline burn is 14.something/1 and ethanol is 12.something/1, brain can calculate the ethanol % in the feedbag, right? Right, provided are all fit and fine. Some when all goes well, the brain can remember the %, and send suitable shots of fuel through the injectors at startup, when “open loop” is needed because the O2 sensors are still cold and senseless. The car within a couple minutes warms the O2 sensors enough to go into “closed loop,” and now the brain has the feedback it needs feed fuel for exhaust excellence. This as we know is called “fuel trim.”
Apparently, adding about 3 gallons or more of fuel prompts the car to do all this figuring. It’s been claimed that the ignition must be off, too, but I haven’t been able to verify that, but no matter, you should always shut down when fueling anyway, you reckless scofflaws.
So to sum up (thank you for your patience,) if the computer thinks there is E85 in the tank and there isn’t, it thinks it needs more fuel than it does, and will (try to) start very rich. If it succeeds without stalling until it’s warm enough to correct the rich condition with fuel trim, it will recognize that the trim needed is excessive, and set a P0172, “Fuel Trim System Rich.”
So, “WHY would the system think it has E85 when it doesn’t?” Well, if GM know, they aren’t saying. But to sense the % correctly, everything needs to be working, “the MAF sensor, injectors, plugs, catalytic converters, O2 sensors and hoo nose what else,” remember? But they know it happens, albeit rarely, and I’ve found several instances in addition to mine, one of which was a (2012?) Silverado, AND there’s a TSB on it. It doesn’t say much except, reset the % to zero. A workaround that has seen some success is to pump the car full of E85 so it won’t choke on startup.
When the system stores a falsely high %, how hard it is to start depends upon how far off it is. Also, it seems the system at least sometimes gradually corrects itself.
This by no means should be taken to mean this situation is the only reason for a P0172.
Some of what I know about this is from Donbrew, some personal experience, some from the font of all wisdom, the interweb.
First, what is “E85”? Lotsa people think it’s 85% ethanol. Once upon a time, maybe, but not any more. It’s anywhere from 51-83%. And the gas station feels no obligation to tell you how much, and would you believe them, anyway? In any event, a FlexFuel is spoda accept whatever ethanol you pump, from 0-85% ethanol, willy-nilly, no need to be consistent.
So the car needs a way to figure out what % is in the feedbag. Nope, no “ethanol sensor,” per se, in an HHR FlexFuel. Some fuel system honcho at GM decided we don’t need no stinkin’ sensor. We have O2 sensors.
When she’s running, the O2 sensors provide the nag’s brain the signal it needs to send the injectors the correct amount of fuel to get O2 level to a happy place. Now that brain, knowing how much fuel and how much air (thank you, MAF sensor,) knows what A/F made fragrant exhaust. Since a eutectic gasoline burn is 14.something/1 and ethanol is 12.something/1, brain can calculate the ethanol % in the feedbag, right? Right, provided are all fit and fine. Some when all goes well, the brain can remember the %, and send suitable shots of fuel through the injectors at startup, when “open loop” is needed because the O2 sensors are still cold and senseless. The car within a couple minutes warms the O2 sensors enough to go into “closed loop,” and now the brain has the feedback it needs feed fuel for exhaust excellence. This as we know is called “fuel trim.”
Apparently, adding about 3 gallons or more of fuel prompts the car to do all this figuring. It’s been claimed that the ignition must be off, too, but I haven’t been able to verify that, but no matter, you should always shut down when fueling anyway, you reckless scofflaws.
So to sum up (thank you for your patience,) if the computer thinks there is E85 in the tank and there isn’t, it thinks it needs more fuel than it does, and will (try to) start very rich. If it succeeds without stalling until it’s warm enough to correct the rich condition with fuel trim, it will recognize that the trim needed is excessive, and set a P0172, “Fuel Trim System Rich.”
So, “WHY would the system think it has E85 when it doesn’t?” Well, if GM know, they aren’t saying. But to sense the % correctly, everything needs to be working, “the MAF sensor, injectors, plugs, catalytic converters, O2 sensors and hoo nose what else,” remember? But they know it happens, albeit rarely, and I’ve found several instances in addition to mine, one of which was a (2012?) Silverado, AND there’s a TSB on it. It doesn’t say much except, reset the % to zero. A workaround that has seen some success is to pump the car full of E85 so it won’t choke on startup.
When the system stores a falsely high %, how hard it is to start depends upon how far off it is. Also, it seems the system at least sometimes gradually corrects itself.
This by no means should be taken to mean this situation is the only reason for a P0172.
Well said!