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Recently I had a PO303 ( misfire cylinder #3 paired with a 420 code ) It was easily fixed with a spare coil and has not returned after 80 miles of driving. The bad coil was a Denso replacement and I was surprised that it went bad so quickly. Its only been about 40,000 miles. Funny how 3 years ago it was ALSO the coil for cylinder 3. THAT I find strange. On the bright side the throttle response is WAY better now. It wasnt running right and I was getting stumble when going into 2 and 3rd when I mashed the gas. I thought I had a transmission issue, but it did not throw a code until recent. I still have a vibration at idle that I have been tracing for 2 years and after this incident Im wondering if the other coils may be starting to go, but havent thrown a code yet and causing my vibration at idle issue. I also inspected the spark plug and was not fouled, and looked great. Im ready to buy 3 more coils to see what happens and get this vibration issue done ! Thoughts ?

I have read sooo many reports of people having little to no luck when replacing ignition coils one at a time with Japanese engines. I don't have a mechanical reason why, although I often expect most people are replacing one at the time (mileage wise) just when the entire set have run their course. I am on my third set. One of my second gen coils failed and the replacement (via my mechanic) fixed the stuttering issue. But once I found the inexpensive OEM Denso, Toyota boxed coils from the ebay seller out of Miami, I went ahead and put the other three in myself. I just searched and the same seller is still actively selling, but 4 at a time and not individually priced.

Caretaker wrote: ↑Thu Sep 02, 2021 4:48 pm I have read sooo many reports of people having little to no luck when replacing ignition coils one at a time with Japanese engines. I don't have a mechanical reason why, although I often expect most people are replacing one at the time (mileage wise) just when the entire set have run their course. I am on my third set. One of my second gen coils failed and the replacement (via my mechanic) fixed the stuttering issue. But once I found the inexpensive OEM Denso, Toyota boxed coils from the ebay seller out of Miami, I went ahead and put the other three in myself. I just searched and the same seller is still actively selling, but 4 at a time and not individually priced.

I apologize, I didnt mention 3 years ago I replaced them all at the same time. All 4 and all Denso.

To recap, you have a 2003; you replaced all 4 ignition coils three years ago. Was that the first time they were ever replaced?
Now 3 years later, you had an issue with the same cylinder's ignition coil in set #2 as with the original factory set. Is that right?

I was at 140,000 miles when I put in my second full replacement set (the third set including the factory set). While no car part is perfect, I think it is plausible to have a failure of an ignition coil after 3 years (and 40,000 miles). And I'm assuming you put in fresh Denso spark plugs at/near the same interval?

Carbon buildup has a lot to do with most cars with the shakes. Too many computer sensors can also drive an owner mad trying to track down causes. Unless I've been totally duped with Chinese copied parts somehow placed into sealed Toyota boxes, I'm pretty satisfied that there are deals online to replace those ignition coils, where once it was $90 for each and more.

Yes ! all 4 coils were replaced for the first time with Denso units 3 years ago....about 40K. The original cylinder #3 coil pack was faulty and that prompted me to replace all 4 ( 3 years ago ) This week I replaced cylinder coil pack #3 last week. Both times the cylinder #3 went bad. Thanks for taking time to respond