Hello all, My CEL went on two weeks ago. Diagnostic code P0171 "lean condition". The mechanic did a smoke test as he originally suspected a vacuum leak. No vacuum leak but he did find a leaky airbox. He suspected this wasn't the cause of the CEL going on but reset the diagnostic system to make certain. He said he suspected it was the O2 sensor but couldn't say for certain which makes me uneasy. I've already put in $55 for the smoke test and to replace the O2 sensor is $300. And $350 cannot give me the peace of mind in knowing that this is the source of the problem. Any insight into these codes is appreciated. I have a 2001 prius. Thanks,
It would be helpful to know a few things: How many miles are on the car? Any driveability issues? Hesitation? misfires? Service history? (fuel filter?) That said.... Reading straight from TIS, the following "trouble area's" should be looked at. Air induction system Injector blockage Mass air flow meter Engine coolant temp. sensor Fuel pressure Gas leakage on exhaust system Open or short in heated oxygen sensor (bank 1 sensor 1) circuit Heated oxygen sensor (bank 1 sensor 1) Some of these are more likely than others. The tech's initial idea that it is a vacuum leak is a good one. The only way an intake leak would cause this is if it was downstream from the airflow meter. Check to see that your airbox is tight around the throttle body. There's a seal ring that could be leaking maybe. Smoke test should have found this, though. Replacing an 02 sensor would be a good next troubleshooting step, but not for $300 I get that. Inspect the wires on yours real good before you replace it. I believe I have an 02 sensor that I could send you for substantially less than $300 (like $25) if you want to try that. Fuel pressure, exhaust leaks, Fuel injectors and temp sensors can all be inspected or tested discretely. (I don't know what access you have to tools and what your level of expertise is). Eric Adopt A Part 303-296-2211
Why not look at scan tool data? I disagree. It's a wild guess. There is nothing in the OP's post to indicate that the P0171 is caused by a vacuum leak. Could it be? Perhaps. But the P0171 needs to be diagnosed, and you'll need data for that. Replacing an 02 sensor is not a troubleshooting step. It's swapping parts. If a private owner has no means of diagnosing their vehicle, and is willing to toss money at a problem, then swapping parts is understandable. But swapping parts is not troubleshooting. It's not even close. Let's back up. Are you saying that you are engaged in professional auto repair?
There isn't a Toyota dealership in my town. I also have not had luck with Toyota in the past so I really doubt they're going to honestly try to help me diagnose the problem in the most cost-effective manner. I have read a lot about this in the last couple of days and everything seems to be pretty much in sync w/ what my mechanic said which is that this diagnostic code cannot pinpoint exactly what the problem is. It can only diagnose the area of functionality where there is a problem. I did talk to my mechanic about everything I've read and one of the things that came up consistently was the MAF sensor. He agree that there was a good chance that the MAF sensor could be the problem and again reiterated that there was no real way to pinpoint exactly what it was...just trial and error. I've read many testimonies online that also reflect this when it comes to this particular diagnostic code. One thing that I found interesting was at a particular Toyota message board, a Toyota mechanic posted that 9 out of 10 times this code alone reflects a faulty MAF sensor. (this code in conjunction with other code however, usually have a different problem source) So, instead of replacing the O2 sensor, my mechanic cleaned the MAF (considerably less) and now I once again have to wait and see if the check engine light comes on. The cost of replacing the sensor is $290. The sensor itself is about $180. The rest is labor. Also, my car has about 150K miles. Hopefully won't need to buy an O2 sensor because the issue is now rectified but I will let you know if that changes. Thanks for all your feedback.
Sounds like a good plan but I have one more suggestion. We've found our model Prius, the NHW11, has a tendency towards 'sticky throttle plate.' You may want to discuss having the throttle plate area cleaned with your mechanic. Also, you might want to look at getting a ScanGauge to work with your car and provide additional diagnostic data for your mechanic. The nice thing is this unit can also give you more details about how your car is running including an additional, high resolution, mileage indicator. But I get the impression you're not 'technical.' Send me a PM and we can work out some way to get a ScanGauge with the adapter and pre-programmed with NHW11 XGAUGEs to provide diagnostic as well as metrics about the battery. I may be able to include the adapter and pre-loaded XGAUGEs at cost . . . . just a thought. Money back if you return the part in six months. Bob Wilson
I made no representation in the post with regards to my profession. The post asked for information regarding the error code. I provided it. Your opinion that replacing an 02 sensor "is not trouble shooting" and that a vacuum leak is not likely to cause a lean fuel trim condition are interesting, and they may have merit or may not. Debating this is certainly not constructive to the issue the OP seeks to solve so I wont waste the bandwidth to do so. Have a nice day, Eric
Many independent Bay Area shops diagnose and repair hybrids regularly. Luscious Garage in downtown SF and Art's Automotive in Oakland are just two which are often mentioned in this forum. That is true for the code. The technician then diagnoses the actual problem. Those who cannot diagnose will swap parts instead. He is dead wrong. Again, this will not normally be a difficult code to diagnose. I don't doubt it. I disagree. And even if it were true, would the technician simply replace the MAF sensor every time? What happens when he encounters that tenth vehicle, and the new MAF sensor doesn't do the trick? Rather than diagnose the vehicle, your mechanic charged you to clean a sensor and "see if it works". My guess is that your mechanic doesn't know how to diagnose the vehicle.
Replacing an O2 sensor to try and solve a problem is parts swapping, not diagnosis. And I didn't state that a vacuum leak is not likely to cause a lean fuel trim condition. I stated that the P0171 may be caused by a vacuum leak, or it may not. Your own list of possible trouble areas, copied from Toyota, confirms that.
p0171 "lean condition bank 1" Hi Eric, I have a similar issue with a Prius 2007. When I purchased it (used), I put ethanol instead of gas. Then, the checked engine light turned on. I took it to Toyota and the told me to keep the ethanol until it is over an then to fill the tank with gas. After the gas was over, I went back to Toyota and they turned the light off. A couple of days later, it turned on again. I came back to Toyota and they found the p0171 code. First, I had also p0300, p0301 and p0302 codes indicating misfire. Toyota advised me to change the throttle body and that is what my mechanic did (I didn't go with Toyota besause of the price). Then these three codes dissappered but the p0171 remained. My mechanic said that he also cleaned the sensors and checked the vacuum. Everytime I take the car from the mechanic, the light is off. But a couple of days later, it turns on. The car is running great. It has 42000 miles and just passed the Illinois inspection. The only little issue is that, when the engine is still cold, it shakes a little before stopping. And that is when the light turns on. Can you, please, help ? Thanks.
What is a "MAF" Sensor? 2001 Prius with 152,000 miles. Runs fine but the Check Engine light comes on with the 171 code. Replaced catalytic converter, O2 sensor and diagnostic showed that the O2 sensor was not getting appropriate signals from the ECU. Replaced the ECU with a part from another 2001 Prius. O2 signals improved but still did not meet specs. Drove the car home from the shop and the check engine light did not come on, but I am expecting it to light up soon. I don't think that a new ECU will cure the problem. Any suggestions?
The engine still creates vacuum when it runs, which, if there is a leak in the air intake downstream of the MAF sensor, means that unmetered air can cause too much air for the amount of fuel being injected, i.e. lean burn. A dirty MAF sensor will also cause P0171 at high speeds, because more air is passing through the sensor than can be registered. Probably 90% of P0171 are dirty/defective MAF. The MAF sensor is located between the air filter and the throttle body, and looks like a little plastic bulb. CRC makes a nifty spray cleaner for it; don't use throttle body cleaner.
