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Today I performed the following maintenance on my 2007 Subaru Outback 2.5i wagon:

  • Replace air filter
  • Clean throttle body
  • Replace PCV valve
  • Replace all PCV pipes
  • Replace throttle body gasket

The PCV valve in particular was in terrible condition and completely clogged of oil and the throttle body had about 1mm - 2mm of gunk build-up on the butterfly valve.

I performed all of the following and upon starting the car, the CEL was on. I immediately turned the car off and checked every electrical connector, and checked that the throttle body gasket was seated properly. All seemed fine. I then disconnected the battery and let the car sit for about 10 minutes and then I started it again. The CEL light was still on.

I let the car idle for around 15 minutes so it can get up to temp, and it seemed to be idling perfectly fine, but the CEL persisted. I decided to take it for a brief drive and it was totally fine, no odd noises, the engine was smooth and performance was good all through the rev range.

This made me think that perhaps the throttle body and PCV valve were so dirty that, after performing the maintenance, the ECU thinks theres a problem.

Is it possible that performing the above maintenance could trigger the CEL? And is it possible that the CEL is a false negative? What should I do?

EDIT: After turning the car on this morning, the CEL is gone. I guess it was a false negative! I purchased an OBD reader anyway, just in case I'm in this situation again sometime.

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    We really need to know what the car's trying to tell you. Have the code read, else we're flying blind.
    – Zaid
    Jul 1, 2018 at 8:13
  • You sure you got everything connected back properly? Double check your work.
    – Moab
    Jul 1, 2018 at 12:00
  • How did you go about cleaning the throttle? It's real easy to damage the TPS. Also the ECM needs to do a throttle relearn, clear the codes wait 30 sec key off, koeo 10 sec then start.
    – Ben
    Jul 1, 2018 at 12:35
  • Have you tested the reader you bought?
    – Solar Mike
    Jul 1, 2018 at 21:30
  • @SolarMike Not yet, it's being posted to me, should be here by wednesday
    – WeakMech
    Jul 1, 2018 at 21:38

2 Answers 2

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Just a thought. To clear the code, disconnect the negative battery terminal and then mash the brake pedal for about 30 seconds. This should completely drain all power from the ECM keep alive memory. Then reconnect the negative battery terminal and restart the vehicle. Then the code should be cleared. After you begin driving the ECM will start the relearn cycle.

In reference to the maintenance you performed it sounds as if the engine was extremely dirty. It's possible the build up around the throttle plate could have been holding the plate open and made the ECM adjust accordingly. Not likely that should cause a code to set though. I would lean more on the cleaning possibly creating a lean condition that the ECM is now re-correcting.

You can purchase an OBD adapter on Amazon for under $20 and download an app called Torque free or the paid version for $5. With these you can do anything that most Actron OBD code readers will. Its a great tool for looking at live data/recording with graphs, all available data PID's. It will also read/reset/clear OBD codes. Can't beat it for the price!

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  • One: did you think of warning the OP to make sure that any radio code is available BEFORE disconnecting the battery? Two: for a 2007 Subaru will it be OBD or OBDII ?
    – Solar Mike
    Jul 1, 2018 at 21:28
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Read the code to see if it is that one.

Then if it is, clear it.

Then, run it and see if it returns. If it does return you may have damaged it while cleaning.

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  • Unfortunately I don't have a code scanner. Also, do you mean it could be that the throttle body is damaged?
    – WeakMech
    Jul 1, 2018 at 7:49
  • You need to beg or borrow a code reader otherwise you have no clue what the code is for - that's why the codes exist - to help us all locate the fault... It may not be the throttle body damaged - it could be a bad connection or broken wire etc etc - read the codes so we can all know what is going on. I bought a code reader for my car and good value it is too! Without the proper tools....
    – Solar Mike
    Jul 1, 2018 at 8:07

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