Quote:
Originally Posted by mr moose
i have a 96 burban 5.7 4x4 200,000 miles plus that keeps showing a code 420. it will kick the code about once a week or so . mostly on the highway. burban seems to run fine . the code reads catlyst efficiency below threshold ( bank 1 ) any ideas? i just dont want things to get worse. thanks. mark.
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Sounds like bank 1 (left side I think) downstream O2 sensor is giving a reading the ECU doesn't like. Could be a bad upstream O2 sensor, bad downstream sensor, or a bad cat.