2003 Chevrolet Silverado Z71 1500 5.3L Engine with P0135 and P0420 Codes: O2 Sensor Issue and Catalyst Efficiency Problem
I own a 2003 Chevrolet Silverado Z71 1500 with the 5.3L V8 engine. Recently, I've been experiencing two check engine light codes: P0135 (O2 Sensor Heater Circuit Malfunction - Bank 1 Sensor 1) and P0420 (Catalyst System Efficiency Below Threshold - Bank 1). Initially, the vehicle displayed both P0134 and P0135 — indicating no activity from the O2 sensor in Bank 1, Sensor 1. I replaced the O2 sensor in Bank 1 with a new unit. After re-starting the engine, the check engine light turned off immediately without needing to clear any codes. However, the next day, the P0134 code disappeared, but P0135 remained. Additionally, the P0420 code now appears simultaneously. I initially considered replacing the Y-pipe (which includes both catalytic converters) as a potential fix for the P0420 issue. However, I'm confused — why is the P0135 code still present after installing a new O2 sensor? My vehicle has 75,000 miles and is past the 8-year/80,000-mile warranty coverage on catalytic converters, though it's still within the original manufacturer’s warranty period. I’ve contacted Chevrolet, but they are not providing support. I'm seeking clarification: Does a P0135 code necessarily indicate a faulty O2 sensor? Can this code directly cause or contribute to a P0420 code? And if so, what steps should I take to diagnose and resolve the issue — especially since I need to remove the front driveshaft to access the sensor connector?
This is a 2003 Silverado Z71 with 75,000 miles. It's past the 80,000-mile warranty limit for catalytic converters under Chevrolet’s 8-year/80,000-mile coverage, even though it's still within the original vehicle warranty period. I’ve tried contacting Chevrolet support — they don’t offer assistance on this issue.