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Questions and Answers

Toyota Tacoma DTC P0171

Q. Hi, I have a 2000 Toyota Tacoma PreRunner, 3.4 liter engine, automatic transmission and 44,000 miles. About two months ago my Check Engine Light came on. I took it to the mechanic and he said it was running lean bank one. Code P0171. He changed the fuel filter and it was good for about two weeks. Then the light come back on.

Toyota Tacoma DTC P0171

I took it back and he said it was like finding a needle in a hay stack if the fuel filter didn't work. I have been doing a little research myself and most people say a vacuum leak. I can't find a vacuum leak that I can see.

I changed the spark plugs, the fuel filter again, PCV valve, air filter, cleaned throttle body, put some fuel injector cleaner in and nothing seems to work. I disconnected the battery to reset the light and it would stay off and the last two time it didn't come on till just after I got gas. I also replaced the gas cap.

If you have any suggestion please help. I do not know a lot about cars so if possible you want me to try something can you tell me how to do it? If it is explainable or if you know a web site that can show me the procedure.

Thank you...

A. DTC P0171 - System Too Lean (Fuel Trim) has a number of causes. I'll run down the more likely possibilities and you can check them out in turn.

  • Air induction system. Any air leak between the Mass Air Flow sensor and the intake manifold will cause this DTC.
  • Injector blockage. You would need to do an injector balance test to determine which injector(s) are restricted.
  • Mass Air Flow Meter. This should throw a code. You can take it out and clean the inside with electronic switch cleaner.
  • Engine coolant temperature sensor. If the CTS is sending the wrong information to the PCM, it could cause the engine to run lean.
  • Low fuel pressure. You need to do a fuel pressure and volume test to check this possibility.
  • Leaky exhaust system. A leak in the exhaust system can dilute the exhaust gas resulting in a false reading.
  • Open or short in the O2 sensor wiring.
  • Bad O2 sensor.
  • A bad PCM.

A scan tool would be helpful in doing some of these checks.

Additional Information provided courtesy of ALLDATA

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