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2002 Chevrolet Silverado P0133 then P0420 - Catalyst Efficiency Issue After O2 Sensor Replacement

Model: Silverado Fault Code: P0133 Posted: 2017-01-21 16:06

My 2002 Chevrolet Silverado with a 5.3L V8 engine has reached 126,600 miles. I initially received a 'Service Engine Soon' warning light. Using an OBD2 scanner, the first fault code detected was P0133 (Bank 1 Sensor 1 - Oxygen Sensor). After replacing this sensor, the check engine light reappeared within 15 minutes of driving, now showing P0420 — indicating inefficient catalytic converter performance in Bank 1. I'm wondering if the downstream oxygen sensor could be causing or contributing to this issue. It seems like a circular problem: replacing one sensor leads to another code related to emissions control. Is there any technical explanation for this sequence? I haven't scheduled a visit to the garage yet. Are there common causes that might explain both codes? Any diagnostic tips would be greatly appreciated. Thanks, Greg

Related fault codes
P0133P0420
Comments (6)
Anonymous 2017-01-21 16:23

After replacing the O2 sensor, it's important to verify if the sensor is properly transitioning between lean and rich conditions. Use a scanner to monitor the HO2S (oxygen sensor) voltage during transitions — look for fixed lean or rich readings. A possible issue could be a stuck-rich condition. Consider performing a fuel pressure test with the engine off (KOEO), observing whether pressure drops significantly over 5 minutes.

Anonymous 2017-01-21 17:00

If the engine were running too rich, the exhaust pipe would typically emit black smoke. In my experience, there's no noticeable difference in exhaust color compared to before the check engine light appeared — suggesting the issue may not be a severe fuel mixture problem.

Anonymous 2017-01-21 18:00

This sequence isn't as odd as it seems. Use your scanner to monitor oxygen sensor voltages: are they stable at high or low levels? Are they fluctuating normally? Check if the downstream post-catalyst O2 sensor is mirroring the pre-cat sensor readings — this can indicate a failing catalyst or wiring issues. I was assuming you might not have an advanced scanner and were relying on basic tools; these tests help narrow down the diagnosis.

Anonymous 2017-01-23 09:58

I recently tapped the catalytic converter, and it produced a springy or metallic sound. Is this normal? Does such noise indicate a failing catalyst?

Anonymous 2017-01-23 12:00

Avoid tapping on components — catalytic converters are expensive and can be damaged by physical impact. Use your scanner to monitor the HO2S rich-to-lean and lean-to-rich transitions. Are the sensor voltages stable or stuck? This data is critical for accurate diagnosis.

Anonymous 2017-01-30 07:38

The check engine light hasn't come back on since I reset it Monday. I've consulted two mechanics, and they're uncertain whether the catalytic converter is actually faulty. I added a bottle of Lucas fuel system cleaner to my tank at half capacity. I also re-routed the downstream O2 sensor wiring to ensure proper connections. Since then, I’ve driven over 100 miles with no return of the code. I’m now choosing to monitor it further. Could water in the fuel cause P0420? The code is broad and difficult to pinpoint — could moisture in the tank be affecting emissions performance?