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1996 Buick Roadmaster LT1 - P0420 Code Fluctuates: Causes, Fixes & OBD2 Diagnosis

Model: Buick Roadmaster Fault Code: P0420 Posted: 2007-12-13 15:26

Hi everyone! I'm having an intermittent issue with my 1996 Buick Roadmaster LT1 (118,000 miles). The vehicle occasionally triggers Diagnostic Trouble Code P0420 on Bank 1, which appears roughly once every week and then resolves for several days. This occurs specifically during cold weather conditions below 25°F at night. I've maintained the car diligently—regular oil changes, filter replacements, MAF sensor servicing—all components have been properly maintained. I recently replaced both oxygen sensors at the driver's side, but the P0420 code persists. Using an OBD-II scanner, I see the following information: - A/C Refrig: Not Applicable - EVAP System: Not Applicable - Heated Cat: Not Applicable - Catalyst: Not Ready Is this a normal reading? Should I perform an OBD2 driving cycle to get accurate data? Or does this indicate a failing catalytic converter? I'd greatly appreciate any insights or troubleshooting steps from experienced owners of the 1996 Buick Roadmaster LT1.

Related fault codes

P0420

Comments (8)

Anonymous 2007-12-13 15:35

Since you've already replaced both oxygen sensors and there are no visible exhaust leaks, the catalytic converter is likely the root cause. P0420 typically indicates a downstream catalyst that isn't functioning properly—this is primarily an emissions-related issue.

Anonymous 2007-12-13 15:51

I replaced only the driver's side oxygen sensors—was that Bank 1? I've checked for exhaust leaks and found none. The engine temperature gauge reaches about 3/4 but doesn't hit red. The thermostat and coolant are both new. Any thoughts on whether a faulty engine temperature sensor could be contributing to this issue?

Anonymous 2007-12-13 17:51

You mean it's just an emissions problem? Does that imply it’s not serious or doesn’t need fixing? I’m concerned—replacing the catalytic converter might not help if the engine is burning coolant. A new converter could be ruined by ongoing coolant leaks, which would also damage your old one.

Anonymous 2007-12-13 23:59

No, I don’t experience any coolant loss or oil burn. The idle temperature behavior seems odd—engine runs warm but doesn't reach full operating temp. Could a faulty engine temperature sensor be sending incorrect signals to the ECU, causing a rich fuel mixture? That could directly relate to emissions issues.

Anonymous 2007-12-14 13:30

Update: I replaced the engine temperature sensor. I also tested the downstream oxygen sensor signal wires—voltage readings were between 0.7 and 1.3 volts at idle and higher speeds. The upstream (driver's side) sensors previously read around 0.6–0.9 volts. Is this within normal range? Does that still point to a failing catalytic converter? After resetting the code, I took a short drive—after warm-up, the check engine light returned. However, the car runs smoothly with no SES lights. Thanks for your help!

Anonymous 2007-12-14 14:54

If the coolant temperature sensor were faulty, you would likely notice symptoms like rough idling or starting difficulties. When testing oxygen sensor voltages, ensure the engine is fully warmed up. Compare upstream and downstream readings to determine if there's a significant voltage mismatch—this helps confirm whether the catalytic converter is functioning properly.

Anonymous 2007-12-15 04:26

The coolant temperature sensor was inexpensive to replace—I’ve now installed a new one, and it’s no longer a suspect. Last night I drove 50–60 miles on highway, and the check engine light remained off. It seems like the issue only appears under city or stop-and-go traffic conditions—still not certain. How does the EGR valve function in this LT1 model? Older 1980s models had EGR valves that could stick open or closed, potentially affecting emissions.

Anonymous 2007-12-17 14:54

Hello! Still going strong—3 days without any check engine light! I swapped the oxygen sensors: installed new ones in the downstream position and moved older (previously used) sensors to the upstream side. The engine coolant temperature sensor is now brand-new. I also changed the oil and filter because the oil change reminder had been showing for 2,800 miles with old oil. Still testing—temperature remains consistently below 25°F all day and night.