← Back to list

P0420 Code on 2000 Subaru Legacy GT: Diagnosis and Aftermarket Cat Issues

Model: 2000 Subaru Legacy GT Fault Code: P0420 Posted: 2008-08-19 13:44

I own a 2000 Subaru Legacy GT that consistently displays the P042-0 diagnostic trouble code. I've already replaced both oxygen (O2) sensors and an aftermarket catalytic converter, but the issue persists. The vehicle runs smoothly with no other error codes present. According to Subaru's service records, a recall involving ECM reflash was performed several years ago—before I acquired the car. In my troubleshooting process, I've noticed that two different scan tools fail to detect the front O2 sensor. Is this normal for this model year? Since comparing the output of both O2 sensors is essential to determine if the catalytic converter is functioning properly, this limitation makes it difficult to validate the system's performance. I believe my vehicle is equipped with a wide-range type front O2 sensor (6 wires, 4 of which are signal wires). Is there a way to test this sensor independently of the ECM or OBD-II interface? Are there alternative methods to assess catalytic converter function without relying on scan tool readings? Any insights into common failure patterns of aftermarket converters in this model would be greatly appreciated.

Related fault codes
P0420
Comments (3)
Anonymous 2008-08-19 16:23

Several forum users have reported poor performance with certain aftermarket catalytic converters. Regarding your scan tool issue—why can't the tools detect the front O2 sensor? Could you specify which models of scan tools you're using? This seems unusual, especially since most sensors are typically readable.

Anonymous 2008-08-19 19:56

My scan tool is a CarChip Pro from Davis Instruments. My mechanic uses a SnapOn model—exact number not known. Both devices can read most other sensors reliably. The catalytic converter I installed is from Davico Mfg., a one-piece unit with integrated cats and flanges that bolt directly onto the exhaust. It comes with a two-year warranty and appeared to be properly installed.

Anonymous 2008-09-05 10:58

After extensive troubleshooting, I decided to replace the catalytic converter with a higher-quality model believed to have better performance. One week later, the P0420 code has cleared and the vehicle appears to be running normally. I had previously replaced both O2 sensors and the original cat. No other issues have been detected that could explain the failure. How common are aftermarket catalytic converters failing shortly after installation in 2000 Subaru Legacy GTs?