I have a 2008 Denali XL. At about mile 81,000 I got the lovely and famous P0420 (Catalyst System Efficiency Below Threshold (Bank 1)) code, with the obvious CEL on the dash.
- I cleared the computer, hoping there was just some anomaly in the system. 2 days later, it comes on again.
- I cleared it again and replaced the downstream 02 sensor. One week later, it comes back again (100 miles or so).
- I get desperate and try the Cataclean. Expensive stuff, for sure. I follow instructions exactly. I did NOT get a chance to reset the code.
- About 50 miles later check engine light is off (meaning the light on the dash is gone but I didn't reset any code). I figured the cataclean worked and I'm shocked, since I've long felt that kind of stuff is more snake oil than science (sorry to offend some of you). But I'm left wondering if codes can just go away. I thought you had to reset them.
- About a two weeks later and maybe 4 to 500 miles more, light and same error code comes on again.
- We drive it with the light on for at least 3 weeks (1000 miles due to road trip) and suddenly the code is gone.
- Drove with it off for about 300 miles, then last night it comes on again.
So it cycles on and off. Stays on more than off but it does all this without me resetting it.
I can't stand the thought of dropping $800+ for new catalytic converters knowing that this code goes away on its own every now and again.
What do I do to figure out the real cause before I start throwing cash at replacing both converters?
- I cleared the computer, hoping there was just some anomaly in the system. 2 days later, it comes on again.
- I cleared it again and replaced the downstream 02 sensor. One week later, it comes back again (100 miles or so).
- I get desperate and try the Cataclean. Expensive stuff, for sure. I follow instructions exactly. I did NOT get a chance to reset the code.
- About 50 miles later check engine light is off (meaning the light on the dash is gone but I didn't reset any code). I figured the cataclean worked and I'm shocked, since I've long felt that kind of stuff is more snake oil than science (sorry to offend some of you). But I'm left wondering if codes can just go away. I thought you had to reset them.
- About a two weeks later and maybe 4 to 500 miles more, light and same error code comes on again.
- We drive it with the light on for at least 3 weeks (1000 miles due to road trip) and suddenly the code is gone.
- Drove with it off for about 300 miles, then last night it comes on again.
So it cycles on and off. Stays on more than off but it does all this without me resetting it.
I can't stand the thought of dropping $800+ for new catalytic converters knowing that this code goes away on its own every now and again.
What do I do to figure out the real cause before I start throwing cash at replacing both converters?
