Code PO171, fuel consumption high?

Tiny
JEFF HERMAN
  • MEMBER
  • 1999 TOYOTA COROLLA
  • 1.8L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 160,000 MILES
I'm getting less than 25 MPG with this car on the highway. I should be getting 35 or better. PO171 code among other evap codes. Live data reads.25v bank1 sensor 1 and no output voltage bank1 sensor 2. Fuel trim sensor 1, 19%. Fuel trim sensor 2, 99%. I don't hear and can't locate a vacuum leak. I'll keep looking with some carb cleaner. As far as I can see there's no MAF sensor on this vehicle. Should there be output voltage from both sensors at idle? Lastly there's a buzzing sound when I try to accelerate hard. It happens when I'm going faster than 25 but without the transmission downshifting, if that makes any sense. I also don't really understand fuel trim numbers I'm just relaying info. The output voltage for sensor 2 isn't within the parameters. I drive a lot and need better mileage, or I might as well drive my CRV. Lol.
Saturday, October 28th, 2023 AT 10:10 AM

74 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,176 POSTS
Hi,

In a perfect world, short-term fuel trims would be 0%. When the percentage goes up, the computer is trying to add fuel to the engine. When it goes negative, it is trying to reduce the amount of fuel to the engine. As I mentioned, in a perfect world, trims would be 0, but as long as they are within +10% and -10%, it is considered normal.

I suspect the oxygen sensor that has no signal is causing the issue. When you look directly at the exhaust manifold, you will notice there are two converters. The oxygen sensor related to the one on the right-hand side is bank 1. Sensor 2 will be after the catalytic converter.

I attached a pic below to help you identify the correct sensor.

Let me know if this helps.

Take care,

Joe

See pic below. Note: I forgot to mention, the voltage you have from the one sensor is within specification. The voltage should run between 0.1 and 1.0v and will vary.
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Saturday, October 28th, 2023 AT 10:00 PM
Tiny
JEFF HERMAN
  • MEMBER
  • 486 POSTS
The set-up is different. There is one sensor in the downpipe which I believe is #1 and there is one post cat which would be #2.
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Saturday, October 28th, 2023 AT 10:19 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

That is very possible. I'm going off what my manual shows. As far as the problem, make sure the connector is in good condition and the wiring isn't melted or shorted.

Let me know if you have other questions.

Take care,

joe
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Saturday, October 28th, 2023 AT 10:42 PM
Tiny
JEFF HERMAN
  • MEMBER
  • 486 POSTS
I have to look again tomorrow but the sheath around the wiring near the sensor is open. It looks like it may have melted but I have to get a closer look. Maybe I can just repair the wiring. As for the connection, it's buried under the carpet and the console, so I have to figure out how to get to it.
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Saturday, October 28th, 2023 AT 10:46 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,176 POSTS
Hi,

Sounds like a plan. Let me know what you find with the wiring that appears damaged. I'm interested in knowing. And certainly, if possible, repair it.

Take care,

joe
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Sunday, October 29th, 2023 AT 6:26 PM
Tiny
JEFF HERMAN
  • MEMBER
  • 486 POSTS
The wiring was intact. The two heater wires read 10.8 ohms. There was no output voltage cold or when operating temperature was reached. After the operating temperature was reached, I checked and there was no resistance at the heater pins of the connector. I also tried bench testing for voltage with a propane torch and got no reading. I proceeded to take apart the under dash and center console and peel back the passenger side carpet just to access and remove the wiring. Ordered a new Denso sensor. I read of trouble with aftermarket ones. I hope this restores my mpg because this was a PIA.
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Sunday, October 29th, 2023 AT 6:41 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,176 POSTS
Thanks for the update, Jeff. I totally understand it was difficult. I have no idea why connectors for the sensors need to be inside the vehicle.

Regardless, let me know if the new sensor takes care of the issue. I think it will and am interested in knowing.

Take care,

Joe
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Sunday, October 29th, 2023 AT 9:00 PM
Tiny
JEFF HERMAN
  • MEMBER
  • 486 POSTS
Update: I installed the new O2 sensor. It was supposed to be a Denso but I'm not sure if it is. Output voltage of the sensor is around.9V Long term Fuel trim sensor 1 is 29% and sensor 2 is still 99%. I don't really know what that means. However, now there is a new code. PO125 Could the new O2 sensor throw this code, or should I be looking at the ECT and the thermostat? The temperature gauge reads normal, about 1/2 way on the gauge.
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Saturday, November 4th, 2023 AT 6:11 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

The new code is helpful. It is related to the ECT. If the ECT isn't reaching operating temperature within a given amount of time. As a result, the system won't go into a closed-loop operation. That is why we are seeing the 99%. The PCM isn't using the information from that sensor because it is still operating in open-loop.

(Closed Loop indicates the PCM is using all the different sensors for the most efficient operation.
(Open Loop indicates the PCM is running the engine in a pre-determined setting while it reaches operating temperature.

I can't guarantee it, but based on what I am reading, it sounds like the ECT may be faulty or is providing a delayed response. Please understand that this is a theory at this point.

Let me know your thoughts.

Joe

See pic below.
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Saturday, November 4th, 2023 AT 8:42 PM
Tiny
JEFF HERMAN
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My question is, was the original O2 sensor failed since there was no output voltage?
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Saturday, November 4th, 2023 AT 8:54 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

If there was input power but no output, yes it failed.

Let me know.

Joe
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Saturday, November 4th, 2023 AT 10:55 PM
Tiny
JEFF HERMAN
  • MEMBER
  • 486 POSTS
I replaced the ECT, and all seemed well. The fuel system was CL. I drove 30 miles with no CEL. When I got home, I hooked up the scanner and it read fuel system OL. I checked for codes, and it was pending PO171. I'm including screen shots of the live data with the OL. Then I cleared the codes, and it was back to CL. I don't really understand short term and long term fuel trims.
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Sunday, November 5th, 2023 AT 2:20 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Fuel trims seem confusing, but they are actually simple. In a perfect world, the trims would stay at 0%. When it is 0%, the computer doesn't have to compensate, and the vehicle is running at its most efficient point.

Now, when (for example) there is an engine vacuum leak, which causes unmetered air to enter the engine combustion chamber. So, you have a lean fuel mixture. The oxygen sensors identify a lean mixture and the PCM makes adjustments to compensate.

For example, if the mixture is lean (too much air and not enough fuel), the PCM forces the injectors to remain open for a longer period of time so more fuel can get into the engine and compensate. That is identified in trims that are in the positive range. For example 5% and any number as long as it is in the positive side of 0%.

When the opposite happens and there is too much fuel and not enough air, the fuel trims go negative because the PCM is trying to lean the mixture. In other words, the injectors are given less time to inject fuel.

What does this all mean? Well, if you are anywhere between -10% and +10%, things are operating in what is accepted as a normal range. When those limits are exceeded, there is usually a problem.

I hope that made sense. LOL

Let me know.

Joe
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Sunday, November 5th, 2023 AT 9:08 PM
Tiny
JEFF HERMAN
  • MEMBER
  • 486 POSTS
Okay, so, now I understand fuel trims. What is long term and short term? What do you make of the readings I sent and what do I do next? No CEL but PO171 pending.
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Sunday, November 5th, 2023 AT 10:32 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

The difference between short and long-term fuel trims is forward. Short-term and happening at the moment the vehicle is being checked. Long-term are over a period of time.

As far as the readings, since the code is pending, it may or may not reset. I recommend driving it for a while to see what happens. The 99.2% fuel trim is an indication the PCM isn't using the data from the O2 sensor, so hopefully, that will change after a drive cycle is recognized.

Let me know.

Joe
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Monday, November 6th, 2023 AT 5:27 PM
Tiny
JEFF HERMAN
  • MEMBER
  • 486 POSTS
I drove about 90 miles. Do you think this all came about with a non-Denso O2 sensor? Now I have codes PO125 and PO171. I didn't have those before replacing the downstream sensor. Drivability is fine and I might have gotten some mileage back. I'm still checking that.
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Monday, November 6th, 2023 AT 6:01 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,176 POSTS
Hi,

If the sensor was an inexpensive one, there is a chance it is faulty. Did you get it online?

As far as the codes are concerned, they are tied together. I attached the diagnostics for the P0125 below. Let me know if you are comfortable performing them.

Joe

See pics below.
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Monday, November 6th, 2023 AT 7:02 PM
Tiny
JEFF HERMAN
  • MEMBER
  • 486 POSTS
First of all, yes, I bought it online and it was sold as genuine Denso. So, I'm looking for the output voltage of sensor #1 to go greater than.45v to indicate a bad ECM? If it stays under.45v suspect wiring or connector issue? There's no misfire. Then check for exhaust leak. No leak replace sensor #1? Read freeze frame data after 3 times 4,000 RPMs, warm engine?
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Monday, November 6th, 2023 AT 7:33 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Yes, yes, and yes. LOL You got it together better than most mechanics. Let me know the results or if I can help in any way.

Take care,

Joe
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Tuesday, November 7th, 2023 AT 5:54 PM
Tiny
JEFF HERMAN
  • MEMBER
  • 486 POSTS
I'm really doubting whether this is a genuine O2 sensor. I also did some testing on my original ECT, and it was definitely responsive to heat on a bench test. The ohms reading seemed a bit low. I never had the PO125 and PO171 codes before the new O2 sensor. Thinking of ponying up for a genuine or even snagging one from a parts car and starting over. When this problem started, I noticed a buzzing sound when kicking down the accelerator. Definitely not the normal acceleration.
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Tuesday, November 7th, 2023 AT 7:08 PM

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