← Back to list

2000 Toyota Tacoma PO420 Code: Catalyst System Malfunction After Cat and O2 Sensor Replacement

Model: 2000 Toyota Tacoma Fault Code: P0420 Posted: 2007-10-13 06:01

Hi, I own a 2000 Toyota Tacoma that has been throwing up a PO420 diagnostic code (Catalyst System Efficiency Below Threshold - Bank 1). I've already replaced both the catalytic converter and the oxygen sensor (also known as the fuel ratio sensor) located before the cat. The vehicle runs smoothly, with no noticeable exhaust leaks or poor engine performance. I used a ProScan device to monitor the readings after replacement — the bank 1 O2 sensor showed improved response, and the code initially changed from 'Catalyst Below Threshold' to 'Catalyst Low'. However, the PO420 code returned after approximately 100 miles of driving. The second catalytic converter appears physically intact with no visible damage or exhaust leaks, and the spark plugs show a clean burn pattern. I'm wondering if there are any other potential causes or troubleshooting steps I should consider? Any experience from others dealing with this issue on a 2000 Toyota Tacoma would be greatly appreciated. Thank you! Jim

Related fault codes

P0420

Comments (4)

Anonymous 2007-10-13 07:51

Is this the same vehicle you've posted about multiple times before? Are these posts related to your own truck or vehicles of family members? I noticed you previously mentioned modifying the downstream O2 sensor position on a Toyota Tacoma — did you install the correct factory-rated catalytic converter during that repair?

Anonymous 2007-10-15 08:52

Just to clarify, I'm not running a business — all the vehicles discussed are family cars. I've worked on cars for most of my life and am simply trying to help others resolve issues. The Toyota Tacoma has been particularly stubborn with the PO420 code despite spending nearly $500 on parts and having several trusted friends assist. I'm not seeking profit, just practical solutions — any advice would be much appreciated. Regards, Jim Brown

Anonymous 2007-10-15 08:57

The first catalytic converter has already been replaced. The downstream O2 sensor was repositioned forward to match the original pipe location since the original sensor is no longer available. One suggestion I received from a friend is to check if the catalytic converters are heating up properly — some vehicles require the cat to reach operating temperature before it can pass emissions tests. This might help confirm whether the converter is functioning correctly. Thanks, Jim Brown

Anonymous 2007-10-15 09:06

It seems unlikely you couldn't purchase a genuine factory catalytic converter from your local Toyota dealer, especially since the truck is only six years old. I'd recommend contacting the dealership to check if there's a software update available that adjusts the PO420 testing parameters — many manufacturers offer such updates. Additionally, an exhaust leak upstream of the O2 sensors can trigger this code, as can other system malfunctions that may degrade new converters. Also, some aftermarket converters are not designed to pass emissions tests and may fail to resolve the PO420 issue.