Three months ago, I had a hard time to start the engine and the check engine light came on. I was told to replace the oxygen sensor and four sparking plugs (Code 135 and 171).
I did so and did not have the ignition problem anymore. However, after a while the engine light came on and off again and again. I went to the same place for repair and they told me the code was 171 but they could not find anything wrong.
Since nothing wrong with the car, i just drove it back. But last week, i couldn't feel the power of the engine while i was driving. When i pulled over and wanted to restart it, i realized it was hard to start the engine. When it was started, i could see the the engine speed is abnormal (faster and then slower repeatedly) from the tachometer.
Anyone knows what could be the problem?
Thanks

 

Have it scanned again, and post the code(s) here.

There might be a short in the wire between the A/F sensor and the ECU, or between the A/F relay. (it's behind the glove box) You might get lucky and only need a new relay.


Was the engine idle surging and dropping, or was it only the tachometer doing this?



P0171 - Fuel system too lean:

1.Start with checking the fuel pressure. It should be 40 - 47 psi.
Is it within spec?
Yes - go to step 2.
No - inspect these items:
If it's too high, replace the fuel pressure regulator. If it's too low, check the fuel pump, the fuel feed line and the fuel filter. (filter is actually inside the gas tank)

2. With the car in park or neutral, start it and raise the RPM to 3000 and keep it there until the radiator fan comes on.

3. Check the output of the primary o2 sensor.
Is the output less than 0.3 volts or more than 0.6 volts?
Yes - Replace the primary o2 sensor.
No - Go to step 4

4. Turn the ignition switch off.

5. With a vacuum pump / gauge, 0-30 in.Hg, apply the vacuum to the EVAP canister purge valve from the intake manifold side. If it doesn't hold vacuum, replace the EVAP canister purge valve.