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Skyvisions, ASE Certified Technician
Category: Toyota
Satisfied Customers: 14324
Experience:  Toyota Master Diagnostic
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2001 Toyota Prius 153,000 miles. Error code P0171 Lean Exhaust.

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2001 Toyota Prius 153,000 miles. Error code P0171 Lean Exhaust. Replaced catalytic converter, O2 Sensor in exhaust manifold, ECM. Scan indicates proper operation but P0171 comes back.

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When you say that you did a computer scan of the computer and everything shows as normal what you mean by that. Did you monitor the long and short fuel trim percentages? What are the numbers?

Customer: replied 4 years ago.

My mechanic and I did a test drive while he monitored the operation of the exhaust with his scanner and it showed normal operation. On my way home the Check Engine light came on. I returned to the garage and he read the code P0171. I don't know the details of his monitoring, but he said that it indicated normal operation whereas with the previous ECM it did not. I can get an answer to your question later today. It is 7:18 am here now.


That will be fine. I will have limited access online today. Do not leave a time request to reply back to you and I will get to you as soon as possible back online. Specifically find out if they have the long and short fuel trim numbers and percentages. They should know what this is. This code can only be set when the oxygen sensors are detecting that there is too much oxygen or air to fuel ratio is to lean. There are two things that are the most common the first one is the mass airflow sensor in the side of the throttle body. The resistor wires will have a tendency to get contaminated. This drives the system lean. Usually this can be cleaned with carburetor and choke cleaner and blow dry with relatively good success. If it continues to happen the sensor will eventually have to be replaced. Have your mechanic remove the mass airflow sensor and look into the plastic extension tube at the resistor wires and see if they look fuzzy or dirty. Have them check for intake leaks and check the ethanol content of the fuel. Let me know what you find.
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Customer: replied 4 years ago.
Replace the downstream O2 sensor, MAF sensor and P0171 still appears. Often it appears when the car is coasting or running in electric mode. I have decided that the problem is somewhere else and as long as the car runs okay I will ignore the check engine light. Thanks for your help. Good advice about a Prius is hard to find.
You are welcome. About the only other thing that I have seen is if the fuel you're using has excessive ethanol content more than 10% by volume it can set a false lean code. Hopefully the mass airflow sensor was a Toyota sensor. I see a lot of problems with aftermarket mass airflow sensors.
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