Code PO171, fuel consumption high?

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.

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.

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

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.

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

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.

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

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.

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.

My question is, was the original O2 sensor failed since there was no output voltage?

Hi,

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

Let me know.

Joe

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.

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

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.

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

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.

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.

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?

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

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.

Hi,

It is very possible that it isn't an OEM part. I've seen people put OEM names on all kinds of things and sell them.

I wish I had an easy answer for you.

Let me know.

Joe

I looked at my old one and it says Toyota and Denso.

So, I did the 3x 4,000 RPMs test and the sensor one voltage never got to. 45v. The highest was about .2v. Did it live data because there doesn't appear to be a freeze option? Also watched fuel trim go from 19% down to about 3-4% when revved and hit zero but not go negative and then return to 19%. I don't really know what any of this means and I'm starting to feel defeated on this one. I would like to try both sensors from a parts car. Point of interest, I was working on a Jeep Patriot so I decided to watch live data. Fuel trim numbers were going from 3% to negative %s on sensor #1. Sensor #2 was same as mine at 99%. I also read that it's sensor #1 that's responsible for fuel trims because it's reading oxygen levels pre- catalytic converter. Could this issue be with sensor #1? The only reason I suspected #2 was because there was no output voltage.

Hi,

You are correct. Sensor 1 is what is responsible for the fuel mixture, but sensor 2 indicates if there is an issue post-catalytic converter. Could it be sensor 1, anything is possible. As far as sensor 2, the 99+% indicates the sensor information isn't being used, so all in all, it could be sensor 1.

Joe

Isn't this fun, lol.

Hi,

Not at all. LOL There are so many variables on today's cars/trucks. My head hurts every day. LOL Let me know how things turn out.

Take care of yourself,

Joe

I'll keep you posted. That's why I like the simplicity of my 64 Dart.

Here we go. I have no ground to the heater of sensor 2. If I create a ground the heater works but still running open loop. Is there an O2 sensor fuse in this car?

Hi,

I attached a pic below showing the power supply and grounds. What I'm seeing is the sensor 2 heater ground is a chassis ground via the ECM.

Take a look. If you have no ground, either we have an open in the wiring, a faulty ECM ground, a damaged connector pin at the ECM, or the ECM has failed.

Let me know.

Joe

See pic below.

The ground for sensor 2 is pink. Where does this wire originate? It seems to come from a main group of wires under the dashboard. Is it supposed to come from the ECM? If I can talk the boneyard into letting me try an ECM from another Corolla of the same year, do I have to reflash it? Also, the heater is turned off by the ECM when the exhaust gas reaches 600F so would/could no ground cause this issue? Is it more likely a failed ECM? Kind of lost here?

Hi Jeff,

The ground is provided via the ECM. Without ground, the circuit can't be completed. If the ECM is grounded but doesn't supply a ground path, there is a good chance the ECM has failed.

The pink wire runs between the sensor and the ECM. The ECM is located under the dash, so yes that makes sense.

As far as the ECM, try to get one with the same part number and it should work. There are a few that were used based on where you are located and the type of automatic transmission you have. See pic 2

Let me know.

Joe

See pics below.

Thanks. I'm really not one to just throw parts at a problem. If I run a direct ground to the heater circuit, I still get open loop fuel system 1. What do you make of this?

Hi,

That takes me back to the ect sensor. However, if we have no ground from the ECM, something is likely happening there.

Joe

I can get a used ECU for $20, but I'm trying some other things first. Denso sensor and thermostat. Is it the temperature sensor on the t-stat housing or the one under the 1,4 coil that sends information to the ECU?

I was just reading that large leaks in the evap system can drastically affect fuel efficiency. My evap lines are basically rotted away in back by the tank and have holes under the driver's area. Could this cause any other codes to be set off other than po455 which I don't have. The only code I keep getting is po125. Thanks

Hi,

According to the manual, the ECT sensor requires the removal of the coil. I attached a pic of what I'm seeing.

As far as the evap is concerned, yes it could cause a lean fuel mixture, but that would be setting additional codes.

Let me know.

joe

See pic below.

That's the one I replaced but now I put the original one back in. Wish this car had a Schrader valve on the fuel rail instead of needing a special adapter to test fuel pressure and see if I might have a leaking injector.

I have some new info. I do have a ground to the heater from the ECM when the engine is running. I put the old O2 sensor back in and once again have no voltage output. I also put my ECT back in and compared it to the new one and they were basically the same. I checked ohms readings in the wires from the ECT. I got the following values on a basically hot engine. Between the yellow/green wire and the white wire there was 550ohms, between the brown and white 500ohms, and between the yellow/green and brown 50ohms. The coolant temperature was 97 degrees C so plenty hot to run closed loop. The code is PO125. I don't think the ECT info is going to the ECM. I'm going to check ohms readings in those wires at the ECM if I can find them. Do they run the same colors to the ECM? Please let me know what you make of this new info. Thank you.

Hi,

I attached the directions below for testing. It really doesn't appear to be too bad. Not the easiest thing to check, but doable.

Also, I forgot to comment on the 64 Dart. I couldn't agree more. Slant 6 or 273? I miss my 1968 Superbee. LOL Those days are behind me now.

Take a look through the directions and let me know your thoughts.

Take care,

Joe

See pics below.

225 Slant 6. Push button Trans, ragtop 270 model. Not perfect but runs good and simple.