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1999 Chevrolet S-10 P0420 Code: Causes, O2 Sensor Location & Diagnostics with Multimeter

Model: Chevy 1999 S-10 Fault Code: P0420 Posted: 2008-08-01 12:32

I recently installed a new catalytic converter in April ahead of an emissions inspection. Initially, the truck passed with no issues and the P0420 code did not appear. However, the P0420 code returned several weeks later. The new catalytic converter is shiny, shows no visible leaks upstream or downstream, and was purchased from a reputable online supplier known for high-quality exhaust components (I’ll verify receipt details soon). I only have access to one O2 sensor located after the catalytic converter. My questions are: 1. Where exactly is the upstream O2 sensor located in a 1999 Chevrolet S-10? 2. Do O2 sensors have internal heaters? If so, which pins should I test using my digital multimeter (I don’t have an oscilloscope but do have an Actron code reader)? The truck has no visible symptoms such as black smoke or backfiring. The crossover pipe broke in two just below the upstream O2 sensor on Friday—this was repaired by welding today. I couldn’t detect any audible exhaust leaks, but I’m wondering if this crack could be contributing to a false P0420 reading. I’ve already invested several hundred dollars repairing the timing chain tensioner failure that caused a complete timing jump, and my budget is very tight. Any guidance on diagnosing or resolving this issue would be greatly appreciated. Best regards, Jack in Atlanta

Related fault codes
P0420
Comments (2)
Anonymous 2008-08-01 20:27

A front O2 sensor does not typically trigger a P0420 code. If the fault is intermittent, consider the quality of the catalytic converter installed—low-cost or poorly manufactured units can cause false readings. Are there any engine symptoms such as misfires or black smoke from the exhaust? This could point to underlying issues beyond just the catalytic converter.

Anonymous 2008-08-02 17:32

No visible smoke or backfiring from the tailpipe. I inspected the catalytic converter visually and it appears clean, intact, and fully open with no signs of damage. The crossover pipe broke near the upstream O2 sensor location—this was repaired by welding today. While I didn’t hear any audible exhaust leaks during inspection, a crack in this area could potentially allow exhaust gas to bypass the converter, leading to an inaccurate oxygen reading and triggering the P0420 code. I purchased the catalytic converter from a well-rated online supplier specializing in OEM-style converters; I’ll check if I still have the receipt for verification. Thanks for the helpful input.