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rpmdiag, Tech and Instructor
Category: Toyota
Satisfied Customers: 7876
Experience:  Since 1975
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I have a 99 4runner. Just recently had the CEL come on with

Customer Question

I have a 99 4runner. Just recently had the CEL come on with a P0420 code. I am also getting a misfire sometimes. Not sure if it is O2 sensor or the Cat. Would a bad sensor cause a misfire?
JA: Does the light stay on, or does it blink?
Customer: It has been on for 2 days now.
JA: Are you fixing your 4Runner yourself? What have you tried so far?
Customer: I would like to try myself. Haven't done any repairs yet. Was going to replace O2 sensor this weekend.
JA: Anything else you want the mechanic to know before I connect you?
Customer: No thats it.
Submitted: 10 months ago.
Category: Toyota
Expert:  rpmdiag replied 10 months ago.

Hi, I'm Ron and I will be helping you today.
Stand by for a moment while I prepare your answer.

Expert:  rpmdiag replied 10 months ago.

The rear 02 sensor needs to be tested with a lab scope. If it's good,then the CAT is bad. No sensor will cause a misfire. Misfires are are from bad ignition, fuel or compression.

Thanks, Ron

Customer: replied 10 months ago.
That being said, changing the sensor if it's bad willl not take care of the misfire?
Expert:  rpmdiag replied 10 months ago.

That's correct.

Thanks again and please remember to rate me 5 stars and keep this link in case you need me again later on.


Expert:  rpmdiag replied 10 months ago.

If you are satisfied with my answer, I would greatly appreciate a 5
star rating now and I will still assist you after you have rated me
too if another question pops up. I don’t get credit for helping you
until you rate me.
Thanks again, Ron