Engine Rough..cat Turning Red..codes P0300, P0303, P0304

[cwel]

HELP!!

The check engine light came on in my 99 RX300 about a month ago. A trip to AutoZone and they told me the codes didn't point to anything specific. Well it was running great so I just kept driving it. All seemed well until a couple days ago the engine started to run rough and the CEL started to flash. I brought it back to AutoZone and they said it has codes P0300, P0303 & P0304.

I also noticed a strong odor coming from the car and found the front cat converter was very red inside it's shielding. The other 2 didn't appear red but it could just be no gaps in shielding to view.

Doing some online research it appears this may be spark plugs or coils. I bought a set of Denso plugs and have initially replaced only plug 3 & 4 (I want to get the problem fixed and then I'll set aside later this week the half a day it appears it will take to get the others replaced). This seemed to make the engine a bit smoother but still rough and in the 1 mile I drove it the CEL now alternates between flashing and steady on. Also from the 1 mile drive my cat converter turned red again. My plan is to now swap the coils with other coils and see if the problem jumps to the new locations for the 3 & 4 coils.

Can anyone advise me on this?

Is my approach to correct and diagnose this the right one so far?

Any other suggestions?

I would also like to note the AutoZone guy stated these were the only codes shown now. I do not understand this as the previous codes weren't reset and although I don't remember what they were, they were not for a misfiring engine. Why would they not still be in the code history since there was no reset and the CEL light was on since the first visit?

Any help would be much appreciated.

CW

[PK_3]

Don't drive it.

Your catalytic converters are burning (I do mean burning as in fire) unburnt fuel inside it. Your car is most likely running to way to rich. A catalytic converter is supposed to clean up a very small amount of residual fue.l You should have had a CEL quite awhile ago. The cause is probably a sensor or more, maybe o2.

Unless you would drive it with a bond-fire in the back, don't drive it till it's fixed.

Regards, PK

[cwel]

Don't drive it.

Your catalytic converters are burning (I do mean burning as in fire) unburnt fuel inside it. Your car is most likely running to way to rich. A catalytic converter is supposed to clean up a very small amount of residual fue.l You should have had a CEL quite awhile ago. The cause is probably a sensor or more, maybe o2.

Unless you would drive it with a bond-fire in the back, don't drive it till it's fixed.

Regards, PK

Thanks PK for the reply, how do you suggest I diagnose and repair the problem?

CW

[cwel]

Just wanted to add an update.

I had placed the potentially bad coils (#3 & #4) to the #2 and #6 locations (wanted any potentially bad coils in front) and put all new plugs in.

I found a friend with a reader and checked following the coil location swap. The new codes indicated the #1 & #2 cylinders were misfiring now, previously was #3 & #4. For those that aren't familiar 2,4,6 are in front and 1,3,5 are in the rear.

I remembered reading that multiple cylinder misfire codes could show up even when only one has a problem due to the bad one affecting the performance of others. Since it appeared that only the front cat was turning red, I suspected that my #1 position coil which I hadn't moved was now being affected by the moved #2 position coil which came from the #4 location (with #4 previously affecting #3).

Changed the #2 coil with coil from Toyota dealer, it was the same price ($95) as my local Lexus dealer but about 10 miles closer to me.

All is now well, runs smooth and CEL is off.

As an added bonus I saved even more by not erroneously replacing 2 coils. I would suspect any shop (dealer or independent) would have showed me the original codes and changed 2.

Thank you!