PO 4020 Code

Hi Folks,
I could use your insight and help.
2000 Camry CE, 5 Speed Manual trans with 186,000 on the clock. This car runs great and I want to keep it.
Just got the P04020 code, Catalyst system efficiency below threshold Bank 1.
Spent a few hours online researching this and seems like my initial options are:

  • A gallon of lacquer thinner in a quarter tank of gas (or any commercial product)
  • Replace the downstream O2 sensor
  • Take off the catalytic converter and wash in soapy water

Am I on the right track?
Please let me know your thoughts on how to deal with this.
Thank you for your time.

Add a bottle of this to the gas tank and then fill the gas tank.


Also be prepared to just replace the catalytic converter.

With 20+ years on the clock… it may be time to get a new one.


its possible its just a o2 sensor especially if its never been changed. it also could be a wire to the o2 sensors that is broken or chaffed. I have had a connector to a o2 sensor that was dirty because a power steering pressure sensor was leaking and power steering fluid leaked into the connector. if your code reader has the option to see if the o2 sensors are cycling properly, check that first. you would also see if power is getting to each of them.

The O2 senor after the catalytic converter produces its own voltage from 0-1 VDC

Here’s what voltage signal from the #2 O2 sensor should look like.


Had the same issue. I ended up using 2 bottles of cataclean.

Is it 0420? Thought cat codes were 04XX?
You have it as P04020

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With Toyota 4 cylinder engines, the downstream sensor is the cause about 60% of the time.

Your engine has a pre-catalytic converter.


If this catalytic converter is bad,it can cause engine damage.


Every engine today has a pre-cat. The correct term is warm-up cat. The issue that Pat Goss referred too was common with the Nissan 2.5l engine because the cat was really close to the exhaust valves and that engine uses a lot of valve overlap in place of an egr valve. I have not heard of this issue with Toyotas.