Check Engine Light after Replacing Spark Plugs

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Aristotelian, Dec 29, 2013.

  1. Aristotelian

    Aristotelian New Member

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    First time poster here, although I have gotten lots of useful information as a reader of this board. I purchased a used 2003 Prius about two years ago, currently has about 125k on it. Not having any idea if the previous owner had ever changed the plugs, I decided to put in fresh plugs. I purchased NGK iridium tipped. Everything went fine, car starts and runs as before. However I am getting a check engine light. Has anyone encountered this and/or is this something to worry about?

    I changed them in order, one at a time. In the process I removed the relay box from its bolts. I did use a small amount of thread grease - no more than a drop for each plug, which I was careful to rub around the threads but not the tip. I did not disconnect the battery before doing the change.

    Thanks for any ideas/advice. I would like to avoid a trip to the mechanic if possible!
     
    #1 Aristotelian, Dec 29, 2013
  2. JC91006

    JC91006 Senior Member

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    You can narrow down the guessing by getting the check engine code. It might have nothing to do with spark plugs.
     
    #2 JC91006, Dec 29, 2013
  3. xliderider

    xliderider Senior Member

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    Many car parts chain stores will read your engine trouble codes for free.

    SCH-I535
     
    #3 xliderider, Dec 29, 2013
  4. edthefox5

    edthefox5 Senior Member

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    Probably a misfire code. Cop's are a pain in the butt.

    Given the year of the car it would have been reasonable to replace the igniters also. I would have just too not have to go back in.

    Probably a connection issue between ignitor and plug tip nothing you did wrong. Should have been greased with dielectric. One poster had a pesky misfire that came and went.
    Found the ignitor boot over the plug had hairline crack in from age that you could only see if you pulled on the boot. Was ok till it was disturbed.

    If codes reveal misfire replace the cop's. Only use OEM you can buy from Toyota online and sometimes I see them on ebay new in the box rather cheaply.
     
    #4 edthefox5, Dec 29, 2013
  5. Patrick Wong

    Patrick Wong DIY Enthusiast

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    Yes, you should get the DTC but it wouldn't hurt to make sure the wiring harness connectors are securely attached to the spark igniters.
     
    #5 Patrick Wong, Dec 29, 2013
  6. Aristotelian

    Aristotelian New Member

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    Hi folks,
    The wife just informed me that the check engine light came on a few days ago unrelated to the spark plugs. Sorry for the false alarm. Mods can delete or mark as solved.
     
    #6 Aristotelian, Dec 29, 2013
  7. Patrick Wong

    Patrick Wong DIY Enthusiast

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    I would like to know the root cause of the CEL once you figure that out. Thanks.
     
    #7 Patrick Wong, Dec 29, 2013
  8. Aristotelian

    Aristotelian New Member

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    Patrick and anyone else, the code came back P0420-catalyst system efficiency below threshold (probable cause)/air leak in exhaust/AF sensor error/fuel system fault/faulty catalytic converter.

    Any ideas to diagnose myself, or should I just take it in? I am not noticing any smells or noise. Fuel efficiency hasn't changed noticeably--about 42.0 to 46.0 depending on the drive and the weather.

    At least it isn't related to the spark plugs!
     
    #8 Aristotelian, Dec 31, 2013
  9. edthefox5

    edthefox5 Senior Member

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    Oh contrare!

    That sometimes reflects poor combustion. Is you engine usuing oil?
     
    #9 edthefox5, Dec 31, 2013
  10. Aristotelian

    Aristotelian New Member

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    Not that I know of, but I haven't checked the oil in a while. I'm not noticing any leaks or weird smoke or anything.

    Also, I will add that the car has always felt a little sluggish. It get up to highway speed but takes longer than other cars I have driven but I figured this is just due to being a Prius.
     
    #10 Aristotelian, Dec 31, 2013
  11. Patrick Wong

    Patrick Wong DIY Enthusiast

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    DTC P0420 is pointing to a problem with the catalytic converter or oxygen sensors, as you pointed out.

    Drivetrain sluggishness might be caused by the catalytic converter getting clogged. The Prius is not a sports car but will accelerate briskly to freeway speeds.

    Here is my suggested approach to deal with that:

    1) Does the car exhibit unusual exhaust noise? If no, then a leak probably is not the issue.
    2) You can try replacing the upstream oxygen sensor to see if P0420 will go away. Use the correct Toyota part, not some universal aftermarket.
    3) After doing #2, if P0420 remains, then you would need to replace the catalytic converter which can be quite costly because of the HCAC chamber which is unique to Classic. Maybe you can find a muffler shop that will cut out the old cc and weld in a universal replacement, depending upon your geographic location (this would not be legal in California, for example).
     
    #11 Patrick Wong, Dec 31, 2013
  12. Aristotelian

    Aristotelian New Member

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    Thanks Patrick. No noise or smells. Either way I think will just take it in to have it looked at by an exhaust place. Thanks!
     
    #12 Aristotelian, Dec 31, 2013
  13. Aristotelian

    Aristotelian New Member

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    Took it in today. They say cat is not clogged so their diagnosis is the 02 sensors. Being replaced as we speak. I think they are charging too much, talked them down from over $800 to $640 but maybe that is still too much?
     
    #13 Aristotelian, Jan 9, 2014
  14. JC91006

    JC91006 Senior Member

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    isn't the oxygen sensor just something you unbolt from underneath the car? It's like a spark plug looking thing if I remember correctly. I did it on my 4runner before, took 5 minutes.
     
    #14 JC91006, Jan 9, 2014
  15. Aristotelian

    Aristotelian New Member

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    There are two of them, but yeah, I am sure it is not hard to do and they did come down on the labor cost. Problem is they are claiming the parts are $200 each. Still probably too much but I have already sunk time and money into this shop and just want to get it done. Hopefully it fixes the problem.
     
    #15 Aristotelian, Jan 9, 2014
  16. JC91006

    JC91006 Senior Member

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    They should know which one is bad, they don't have to replace the one that is not bad.
     
    #16 JC91006, Jan 9, 2014
  17. Patrick Wong

    Patrick Wong DIY Enthusiast

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    If you are concerned about the repair cost, replace only the upstream oxygen sensor (located between the engine exhaust manifold and the catalytic converter.) Do not replace the downstream oxygen sensor (located after the catalytic converter).
     
    #17 Patrick Wong, Jan 9, 2014
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