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Rough Idle, 0420 Code - So Many Trips to the Mechanics and No Resolution. Toyota Corolla.

greggietimegreggietime Member Posts: 2
edited September 2014 in Toyota

So, I have a Corolla [about 90,000 miles] that idles rough (rougher when I use the power windows or the AC) and is throwing a 0420 code. Here's what's happened up to this point:

1) First, for years it ran fine and I didn't have a single problem with it. Then it started accelerating sludgily, using gas faster than normal, idling roughly and the 0420 code came up.

2) I took it to my [trusted] mechanic [Mechanic 1]. They referred me to a shop that specializes in catalytic converters [Mechanic 2] and the like for the 0420 code and referred me the dealership for the idle problem. He said that the dealership has more experience with my specific car.

3) I have gone back and forth between the dealership and Mechanic 2 about ten times total. They cannot seem to figure the thing out.

4) The dealership replaced the air fuel sensor and a seal on the mass air flow sensor. [I got sleazy vibes from these guys.]

5) Mechanic 2 replaced the catalytic convertor (and is about to replace it again -- something about a hole in the replacement), both O2 sensors, the spark plugs and ran some kind of cleaners through the fuel system. They seem to be running all kinds of tests, doing research (I sat at the shop, for part of a day, and watched as a mechanic moved back and forth between my car, some devices and a computer, in an increasingly harried manner) and working with someone outside of the shop called the "tech team." [I've gotten good vibes from these guys.]

6) I've cleaned the mass air flow sensor with electronics cleaner, replaced the engine filter and also replaced the battery (it was close to dying).

7) I've dropped $1,800+ on these problems (which is a lot for me).

8) The sludgy acceleration has mostly gone away, but the other two issues are still present. I'm worried that this next fix, re-replacing the catalytic converter, won't work either. (I wouldn't be paying for the re-replacement, but I'm worried about the car.) Mechanic 2 said I'd be able to pick up my car today (it's been in the shop for four days). Also, both my Mechanic 1 and 2 said that, although troubled, my car is okay to drive.

What do I do? [I've begun to fixate.]


  • greggietimegreggietime Member Posts: 2
    edited August 2014

    The 0420 code did actually go away for a while, after the initial repairs, but it came back about half way into a 1,400 mile road trip.

  • Karen_SKaren_S Member Posts: 5,092

    And this Youtube video too.


  • toytech1267toytech1267 Member Posts: 1
    if the cat they put in your car was aftermarket, it will most likely cause the p0420 to kick back on. Toyotas do not generally accept after market parts (esp electronic or exhaust) very well. same goes for the sensors.
    Several Toyotas also have service bulletins for that code and usually require replacement of the cat and a reflash of the computer. Yours is an 07? i will check for T-SB's on monday when i get to work.
    In terms of idle and running rough, after to you replace the battery, you may need to clean the throttle plate off. The computer resets and is expecting to see a clean throttle plate, but after 90k miles there will be build up on it and the computer will have trouble finding idle. in general though, most toyota 4cyl vehicles have some weak idles as they get higher mileage. its a common complaint.

    i have 10 yrs Toyota experience and own an 05 corolla. i will see if here are any bulletins on monday. good luck.
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