Rich Mixture
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Joined: 01-23-2009
Posts: 26,539
From: Fredericksburg,VA
You describe a working #1 O2 sensor.
Alcohol percentage is based on how much O2 is in the combustion gas. That could be from unmetered air, which would also read as lean mixture causing the trim to be too high. P0172.
I know pulpfriction disagrees with this logic; no need to comment.
Alcohol percentage is based on how much O2 is in the combustion gas. That could be from unmetered air, which would also read as lean mixture causing the trim to be too high. P0172.
I know pulpfriction disagrees with this logic; no need to comment.
Last edited by donbrew; Sep 22, 2025 at 05:05 PM.
Thread Starter
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Joined: 02-01-2006
Posts: 1,471
From: lockport,ny
Ok, so the consensus is to check for exhaust leaks (upstream of the O2 sensor) before trying to reset the Alcohol percentage?
It looks like an unmetered exhaust or intake leak causes the computer to think there is a higher level of alcohol than there really is.
The flex joint has been replaced before with a braided one, so its not as obvious if there is a leak there as one the factory "bellows type".
If I have the alcohol percentage reset and there is a leak, wont the computer just drive the percentage high again?
Steve
It looks like an unmetered exhaust or intake leak causes the computer to think there is a higher level of alcohol than there really is.
The flex joint has been replaced before with a braided one, so its not as obvious if there is a leak there as one the factory "bellows type".
If I have the alcohol percentage reset and there is a leak, wont the computer just drive the percentage high again?
Steve
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Joined: 01-23-2009
Posts: 26,539
From: Fredericksburg,VA
Between the MAF and the #1 O2 sensor. The MAF meters the O2 and the sensor senses how much O2. The computer knows how much O2 there should be based on the commanded throttle position.
Could be a crack in the air horn where the PCV tube lets in or in the tube to the brake booster or anywhere else.
Could be a crack in the air horn where the PCV tube lets in or in the tube to the brake booster or anywhere else.
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Joined: 12-05-2014
Posts: 3,371
From: Northern Ohio
May I anyway? [edited for civility]
Consensus doesn't mean majority, it means agreement. There is no consensus here unless unless you ignore my comments. I was frustrated trying to get you to check the ethanol%. When did you learn it was elevated? The fact that is pretty much proves what I was saying all along. (Not the 1st time I was pooh-poohed when I brought up stored ethanol %. Not the 1st time I was vindicated.)
Maybe. But there may be no leak. This is a known issue with many GM FlexFuel cars, not just HHR. Ciuld be a bug in the algorithm or the ECM could be a design flaw. IDk.The GM TSM says to reset the ethanol%. Why not beleive them? No parts needed. If it returns, that's more information.
I'm not against checking for exhaust leaks, of course, before or after resetting. But I don't see how a leak large enough to cause this problem wouldn't be audibly obvious. I don't even understand how anything but a gaping hole would cause this, because the O2 sensor doesn't measure flow. But I'm open to education.
I had this problem. I fixed a fuel line leak and replaced the battery. That did nothing. I reset the stored ethanol % to zero. It found it's way up to something around 10%, consistent with normal fuel around here, and it's been fine for years.
Good luck.
Consensus doesn't mean majority, it means agreement. There is no consensus here unless unless you ignore my comments. I was frustrated trying to get you to check the ethanol%. When did you learn it was elevated? The fact that is pretty much proves what I was saying all along. (Not the 1st time I was pooh-poohed when I brought up stored ethanol %. Not the 1st time I was vindicated.)
I'm not against checking for exhaust leaks, of course, before or after resetting. But I don't see how a leak large enough to cause this problem wouldn't be audibly obvious. I don't even understand how anything but a gaping hole would cause this, because the O2 sensor doesn't measure flow. But I'm open to education.
I had this problem. I fixed a fuel line leak and replaced the battery. That did nothing. I reset the stored ethanol % to zero. It found it's way up to something around 10%, consistent with normal fuel around here, and it's been fine for years.
Good luck.
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Joined: 01-23-2009
Posts: 26,539
From: Fredericksburg,VA
I believe I used the qualifier "could" a couple of times.
We filled 4 pages disagreeing in this thread alone; is there really a need for more?
O2 from burning alcohol reads the same to the O2 sensor as air in the exhaust. The sensors are not smart enough to distinguish. So the computer will report high alcohol if there is unmetered air.
Here is a complete explanation of how alcohol content is calculated note that the purge system might be to blame.
We filled 4 pages disagreeing in this thread alone; is there really a need for more?
O2 from burning alcohol reads the same to the O2 sensor as air in the exhaust. The sensors are not smart enough to distinguish. So the computer will report high alcohol if there is unmetered air.
Here is a complete explanation of how alcohol content is calculated note that the purge system might be to blame.
E85 Flex Fuel DescriptionE85 compatible vehicles no longer use an alcohol sensor to determine and adjust for the alcohol content of the fuel in the tank. Instead, the vehicle calculates the alcohol content of the fuel through measured adjustments.
The ethanol calculation occurs with the engine running after a refueling event has been detected via a measured change in the fuel level sender output. The virtual flex fuel sensor (V-FFS) algorithm temporarily closes the canister purge valve for a few seconds and monitors information from the closed loop fuel trim system to calculate the ethanol content. This logic executes several times until the ethanol calculation is deemed to be stable. This may take several minutes under low fuel flow conditions such as idle, or a shorter time during higher fuel flow, off-idle conditions.
Air-fuel ratios and the corresponding ethanol percentage are updated following each purge-off sequence. The fuel alcohol content percentage value can be read on a scan tool.
When an E85 compatible vehicle is built, an ECM or PCM replaced, or if the learned alcohol content has been reset with a scan tool the fuel system will need to contain ASTM gasoline with 10 percent or less ethanol content.
A minimum of 11 liters (3 gallons) must be put in the tank in order for the vehicle to recognize a re-fueling event. It is not necessary to turn the ignition OFF in order to have the re-fueling event recognized, however local safety regulations should be followed.
After the re-fueling event, the system registers the amount of fuel that was added, relative to the amount that was in the tank. Reading fuel trim and O2 sensor activity, the system determines if the fuel added was either ASTM Gasoline or ASTM E85. Based on that determination, the system adjusts to the expected alcohol mix in the fuel tank, and then the fuel trim and O2 sensor activity fine tunes the adjustments. The system must remain in closed loop in order for this adjustment to occur. Numerous short trips after switching from gasoline to E85, or E85 to gasoline, can result in driveability symptoms due to the inability of the system to adjust for fuel composition by not attaining closed loop
The ethanol calculation occurs with the engine running after a refueling event has been detected via a measured change in the fuel level sender output. The virtual flex fuel sensor (V-FFS) algorithm temporarily closes the canister purge valve for a few seconds and monitors information from the closed loop fuel trim system to calculate the ethanol content. This logic executes several times until the ethanol calculation is deemed to be stable. This may take several minutes under low fuel flow conditions such as idle, or a shorter time during higher fuel flow, off-idle conditions.
Air-fuel ratios and the corresponding ethanol percentage are updated following each purge-off sequence. The fuel alcohol content percentage value can be read on a scan tool.
When an E85 compatible vehicle is built, an ECM or PCM replaced, or if the learned alcohol content has been reset with a scan tool the fuel system will need to contain ASTM gasoline with 10 percent or less ethanol content.
A minimum of 11 liters (3 gallons) must be put in the tank in order for the vehicle to recognize a re-fueling event. It is not necessary to turn the ignition OFF in order to have the re-fueling event recognized, however local safety regulations should be followed.
After the re-fueling event, the system registers the amount of fuel that was added, relative to the amount that was in the tank. Reading fuel trim and O2 sensor activity, the system determines if the fuel added was either ASTM Gasoline or ASTM E85. Based on that determination, the system adjusts to the expected alcohol mix in the fuel tank, and then the fuel trim and O2 sensor activity fine tunes the adjustments. The system must remain in closed loop in order for this adjustment to occur. Numerous short trips after switching from gasoline to E85, or E85 to gasoline, can result in driveability symptoms due to the inability of the system to adjust for fuel composition by not attaining closed loop
Premium Member
Joined: 09-23-2009
Posts: 486
From: Hurst Texas
Just got mine to pass inspection, had a PO171 code for lean condition. Did a smoke test to discover the unmetered air leak in a strange place. It was the vacuum check valve in the brake booster, a very tiny crack where the plastic cracked. Replaced the check valve and all good again, just shows how little it takes to get the code.
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Joined: 12-05-2014
Posts: 3,371
From: Northern Ohio
Bear with me, new thought here.
OK, but there was much discussion about an exhaust leak, but how would unmetered air will enter between the exhaust ports and the O2 sensor? A leak there will expel exhaust, not suck in air. And the O2 sensor knows nothing about volume, only O2 %. It's the only way the ECU can know A/F.
Buuuut, as Don posted, (would love to know the source of this very interest info, btw)
P0172 indicates only that, once in closed loop, computer thinks the needed trim A/F trim is excessive, i.e. the the car started too rich due to a faulty stored ethanol %. So why was it faulty?
Wholly cow! I now recall that shortly after my ethanol reset, the "small evap leak" code popped up (that eventually graduated to a P0255, It proved to be the purge valve. Never caused any other problems, tho.
What if the problem is caused by an undetected leak in the purge valve? That plausibly would result in and error in the calculated ethanol %!
Pretty sure the TSB said something about evap leaks.
For what it's worth.
Buuuut, as Don posted, (would love to know the source of this very interest info, btw)
What if the problem is caused by an undetected leak in the purge valve? That plausibly would result in and error in the calculated ethanol %!
Pretty sure the TSB said something about evap leaks.
For what it's worth.
Last edited by PulpFriction; Sep 28, 2025 at 02:32 AM.
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Joined: 01-23-2009
Posts: 26,539
From: Fredericksburg,VA
The purge valve is a PWM device; lots of ways it could be faulty. Literally 1048 ways
Air from an exhaust leak before the sensor would be unmetered introduced by the venturi effect.
That was a quote of a quote that was not attributed, but I think it is from a GM technical source.
Air from an exhaust leak before the sensor would be unmetered introduced by the venturi effect.
That was a quote of a quote that was not attributed, but I think it is from a GM technical source.