Check Engine + VSC = Code P0420


Asked by Feb 16, 2016 at 07:20 AM about the 2004 Toyota Highlander Base V6 AWD

Question type: Maintenance & Repair

I recently purchased this car, and today the check
engine light and VSC came on. I had Advance Auto
scan it and it came up with P0420. What are some
troubleshooting steps I can do before taking it in to
possibly replace the exhaust or cat? Other possible
reasons for this?

2 Answers


It wouldn't hurt to test fuel pressure to make sure you don't have leaky fuel injectors. unburned fuel can cause this problem too. also, the o2 sensors can be tested. It might not hurt to have the exhaust inspected and "sniffed " by a good muffler shop. this could give you a better picture of what is going on.


as far as the vsc code, you are going to need a more extensive scan than the little code reader that the auto parts stores use. It wouldn't hurt to have a quality shop scan it with their professional equipment. they can read "live data". this means that they can assess while the engine is running and give a good pic of all systems while they are operating.

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