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1997 GMC Safari P0420 Fault: Catalytic Converter Replacement Needed? O2 Sensor Test IDs $70 & $71 Explained

Model: 1997 GMC SAFARI Fault Code: P0420 Posted: 2009-02-22 20:48

My 1997 GMC Safari with a 4.3L V6 engine and single exhaust system has triggered a P0420 diagnostic trouble code (DTC), indicating low catalyst efficiency in Bank 1. The post-catalytic converter oxygen sensor (O2) signal graph closely mirrors the pre-cat sensor reading, ranging from 0.15V to 0.79V — a clear sign of poor catalytic converter performance. After scanning with an OBD-II scanner, only P0420 was detected. However, under the O2 Sensor Monitor (Mode $05), several tests appear out of specification for both Bank 1 Sensor 1 (B1S1) and Bank 2 Sensor 1 (B2S1): - Test ID $70: MAX=255, MEAS=255, MIN=50, MOD=$10 - Test ID $71: MAX=255, MEAS=255, MIN=50, MOD=$10 These values suggest the sensors are not responding properly to exhaust conditions. Additionally, Diagnostic Monitor tests under TEST $02 and TEST $07 show abnormal readings: - TEST $02 (ID 46): MEAS=0, MIN=64, STS=LOW - TEST $07 (ID 0C): MEAS=8000, MAX=0, STS=HI; ID 0D: MEAS=f8af, MAX=9b, STS=HI I’ve been unable to find official documentation explaining these specific test IDs. Are these manufacturer-specific tests? Where can I access reliable information on them? Regarding the catalytic converter replacement, I’m considering brands like Walker, MagnaFlow, and Maremont (pricing ranges from $330–$415). I’ve heard reports that some aftermarket converters don’t perform as well as OEM units, leading to P0420 reoccurrence after installation. Should I opt for an OEM catalytic converter instead? Any recommendations on the best replacement option — especially in terms of reliability and compatibility with my 1997 GMC Safari? Thanks for your insights.

Related fault codes
P0420
Comments (6)
Anonymous 2009-02-22 21:09

Before performing additional tests, confirm whether any other DTCs exist beyond P0420. While an OEM catalytic converter is typically high-quality and reliable, many users have successfully used Walker aftermarket converters without issues. We’ve never experienced problems with direct-fit replacements — just ensure you’re using a model specifically designed for your 1997 GMC Safari.

Anonymous 2009-02-23 15:26

A practical workaround if the catalytic converter is marginal: cut the signal wire from the post-cat O2 sensor and insert a 1-megohm resistor inline (available for ~$0.99 per pack at Radio Shack). This trick fools the ECU into thinking the catalyst is functioning properly, which may clear P0420 temporarily — though it’s not a long-term fix and should only be used if tailpipe emissions are within acceptable limits.

Anonymous 2009-02-23 20:37

I only have P0420 as the active code. The O2 sensor test results I pulled from my scan tool show abnormal values for IDs $70 and $71 — any idea what these specific tests mean in Mode $05? I’m leaning toward a Walker direct replacement, but I’d like to verify if this is safe. Thanks for the suggestion about cutting the post-cat O2 sensor wire. That could be a viable short-term fix if emissions are acceptable and inspection isn’t required — though it’s not a permanent solution.

Anonymous 2009-02-25 17:50

I’m wondering whether official factory service manuals or online subscription services (like those from AC Delco or other OEM providers) contain detailed explanations for Test IDs $70 and $71 under the O2 Sensor Monitor in Mode $05. These may be manufacturer-specific, so I’d prefer to invest in a reliable source rather than waste money on incomplete data. Any recommendations? Thanks again.

Anonymous 2009-02-26 09:44

Mode 6 diagnostic data is available at: http://www.acdelcotechconnect.com/ — this site includes detailed test ID descriptions and interpretation for various OBD-II systems, including those used on older GM vehicles like the 1997 GMC Safari.

Anonymous 2009-02-27 20:02

Thanks for the link! The test IDs I was looking for are now clearly explained in the documentation. I’ve also ordered a factory service manual to ensure full understanding of the diagnostic procedures and sensor behavior on my vehicle.