LanceLink

Code P0420 (Bank 1), 2003 ES300 - Smog test next month. Code Reader ($20 HyperTough HT309, version 1.01) says ‘Codes Found - 0 and Monitors Incomplete - 3’. (Monitors Incomplete will start at 4.) However, if I go ahead and select Read Codes, it shows P0420 and 1/1. After driving another 30 miles, the Check Engine light appears. Now it says ‘Codes Found - 1 and Monitors Incomplete - 2’. It shows 2 duplicate P0420 error codes, 1/2 and 2/2.

After reset, if I can get it down to no codes, no Check Engine light and 'Monitors Incomplete - 1', I'm good to go.

Second owner since 2009, 168K miles, just ran SeaFoam with 8 gallons of gas, no change. In the last 4 months, it has developed a significant valve cover oil leak, due to overheating in 2017. (Aftermarket water pump failure at 50K. Always go with OEM Denso!) Plugs and coils were recently replaced. Any thoughts? Thanks!

Arsenii

Hello,

Code P0420 means that you catalyst is not efficient enough, which, in turn, means that your catalyst or your downstream O2 sensor could be bad. Especially if considered that those codes pop-up only after the engine is warmed.

About the overheat - are you sure that it didn't warp the cylinder head? Is your coolant topped off? Any smoke from the exhaust?
If it is just the valve cover, I would highly suggest replacing both of the gaskets, they are not that expensive, and it is not that hard to do.

P.S. Denso doesn't make water pumps for Toyota, they are made by Aisin.

Hope this helps and best of luck!

LanceLink

Thanks, I plan to have valve cover gaskets replaced this week. Recent engine compression test was fine, coolant always full, no smoke from exhaust.

Arsenii

That is great!

You can actually diagnose the O2 sensor by measuring its output voltage. Here is the article about it.

Hope this helps and best of luck!