P0420 code reappears even after BOTH O2 sensors andcatalytic converter replaced.
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P0420 CEL code appears. Replaced downstream O2 sensor, cleared code ran car @ 130 miles and P0420 reappears. Replaced upstream O2 sensor, cleared code ran @ 75 miles and P0420 code reappears. Put bottle of CataClean additive into half full gas tank and ran non-stop hiway speeds for 150 miles. 2 days later P0420 REAPPEARS! Replaced catalytic converter and ran 140 miles and P0420 REAPPEARS!! Read online with new cat converter to drive vehicle 45 minutes, stop and shut engine off for 1 minute. Restart car and drive vehicle another 20 minutes and shut car off. A few days later, guess what...P0420 code REAPPEARS!!! Engine runs/idles nicely, doesn't seem to have any issues. What's wrong??
How long has your 2017 Buick Encore had this problem?
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Hi this is Dusty, I'm one of the auto experts on RepairPal's Car Genius team.
Sounds like a major headache you've got there; generally speaking, catalyst efficiency codes are usually very straight forward in terms of repair.
You mentioned that both oxygen sensors have been replaced along with the catalyst, which leaves a few possibilities.
The software in the engine control module is obsolete or a glitch and is falsely triggering a persistent fault.
The data from the oxygen sensors is being influenced by an outside source like faulty wiring, or poor connections.
There are other possibilities of course, but having your vehicle diagnosed by an experienced professional will be able to make heads or tails of it.
If you have any other questions feel free to give us a call at 877-913-1554! We'll be happy to help or find you a certified shop in your area!
