I have a Toyota Corolla 1999. The check engine light turns on. The mechanic has replaced brand new 4 spark plugs, downstream oxygen sensor, ( it was reading zero all the time, giving some output voltage after replace). Still after few days, it comes back on. I spent 3 thousand dollars in all these, but no avail. What is the root cause of it? Now two mechanics, one MIDAS, another individual shop do not know the root cause of it. He said scanner is telling no Problem of car computer, but CEL is on.
It might help if you let us know what the specific codes are as well as what live data looks like. ("No problem of car computer" does not tell us anything.)
Also if you have been charged $3000 to replace 4 spark plugs and an O2 sensor that is way beyond being even a mere rip-off.
This is my first car. I do not know what to say.
The problem here is we just don't have any information at all to go on. If you can get us details about what the code or codes were before being cleared, as well as any symptoms the car has been exhibiting, that would help. It's a 25-year-old car so almost anything goes.
The mechanic told me that the scanner is giving error code P0420.
There are many things that can cause a P0420 code, including but not limited to a bad catalytic converter. Has your mechanic checked the items listed and taken the diagnostic steps detailed below?
https://www.walkerexhaust.com/support/tech-tips/diagnosing-catalytic-converter-p0420-code.html
https://www.way.com/blog/p0420-code-what-it-is-and-how-to-fix-it/
https://www.carparts.com/blog/p0420-code-catalyst-system-efficiency-below-threshold-bank-1-2-2/
https://www.obd-codes.com/p0420
I do not know what to do.
Your Amazon link is not working for me, it just says "Kindle - sorry we weren't able to load the preview". It's the first I've heard of a gas cap having the effect of triggering P0420. Typically a gas cap problem would cause a code for the evap system.
Aside from that, your description makes it sound like your mechanic did some cursory checks but didn't really get very deeply into it. If you look at the links provided you'll find a litany of potential causes for the problem as well as troubleshooting suggestions. The videos below are samples of the kind of diagnostic procedures that can be done.
You need to find a better mechanic who is willing to do the necessary troubleshooting. (If you were anywhere near Clarksville, Tennessee you could make an appointment with Scotty to check it out for you.)
https://www.youtube.com/watch?v=CflIvd5VOoA
https://www.youtube.com/watch?v=tEnHS0oC3bo
https://www.youtube.com/watch?v=9VZ5K8n5jj0
https://www.youtube.com/watch?v=zQ8BXAwK-QY
The car is with the second mechanic. He said over the phone that there is no code via the scanner, but he cleared the code after the Check Engine Light (CEL) came on. This is my first car. I do not know what to say.
The mechanic told me that the scanner is giving error code P0420. He replaced a new oxygen sensor at downstream (as far as I can recall). To his opinion, the scanner is providing (correct) voltage. The scanner was always giving zero voltage, before the replacement. Even after replacing a brand new oxygen sensor, the CEL light came on after a week.
The mechanic told me that there is NO engine misfire, spark plugs are fine, the upstream and downstream oxygen sensors voltage are normal, there is no problem in wiring. He does not know the real cause of this problem. I pointed him that the Gas cap is not original Toyota, may be this is causing problem. I pointed him to page number of Toyotal manual, He got angry, and told me not to bother him, he is the expert, not me. I do not know what to do. I even send him links in amazon, people finding a correlation between CEL and gas cap, the mechanic disagreed.