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P0420 Code on 1999 GMC Sierra 5.3L V8: What Does It Mean and How to Fix It?

Model: 1999 GMC Sierra Fault Code: P0420 Posted: 2006-08-03 05:41

I own a 1999 GMC Sierra with the 5.3L V8 engine. Recently, the vehicle began running poorly. After connecting a code reader (TP-150 Code Scanner), I found the following diagnostic trouble codes when the engine was off and key was on: - P0102: Mass Airflow Sensor Issue - P0420: Downstream Oxygen Sensor Failure (Bank 1) - P1441: Fuel Tank Level Sensor Circuit Issue I replaced the mass airflow sensor, which resolved the P0102 code. The P1441 code has not reappeared since. However, I still have a persistent P0420 code — this indicates a problem with the oxygen sensor located downstream of the catalytic converter on Bank 1. My question is: Which side is 'Bank 1'? Is it the driver's side (left) catalytic converter? Additionally, I now see two instances of the P0420 code. Does having multiple identical codes mean something different? For example, does one represent an active fault and another a pending or stored issue? Thanks for your help, Dave

Related fault codes
P0420
Comments (5)
Anonymous 2006-08-10 15:44

I recently had my 2001 GMC Suburban in the shop for a P0430 code on Bank 2 (passenger side), and diagnostics revealed that the oxygen sensor was fine — the issue was actually the catalytic converter. Good news: GM covers catalytic converters under warranty for up to 8 years or 80,000 miles. I recommend contacting your local Chevrolet dealership to confirm coverage. My truck has two catalytic converters, both located on the crossover pipe.

Anonymous 2006-08-10 17:53

Bank 1 refers to the cylinder bank that contains cylinder #1. In most V8 engines, this is typically the driver's side (left) of the engine. The second P0420 code likely indicates one instance is active and another is pending — a common scenario where the ECU stores multiple entries for the same issue. This doesn't necessarily mean there are two separate problems; it may reflect an intermittent fault or a system that has not yet fully resolved the issue. Good luck, and please update if you get further results.

Anonymous 2006-08-12 08:17

Thanks for the clarification, Jeff. Could you confirm whether cylinder #1 is on the driver's side or passenger side in a 1999 GMC Sierra with a 5.3L V8? I’ve seen conflicting information online and want to be sure about which bank I’m diagnosing.

Anonymous 2006-08-12 08:18

The post by 5DOGS is spot-on — I experienced the same issue at exactly 80,359 miles. The P0420 code appeared after a catalytic converter failure on the driver's side. Replacing the converter resolved the problem and cleared all codes.

Anonymous 2006-08-12 12:49

I don’t recall offhand whether cylinder #1 is on the driver’s or passenger side, but generally, the cylinder head closest to the radiator or further forward in the truck will be Bank 1. For a definitive answer, I recommend checking the owner's manual or using a service guide like Chilton’s. As a quick test, you could remove a spark plug wire or fuel injector from one of the front cylinders and see which misfire code appears — this can help identify the side with the issue.