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When we bought the car the engine light was on and dealer said it was O2 sensor. They replaced sensor and light went off for a few hundred miles. Light came back on and code is for catalytic converter. P0420 Cat Sys Efficiency Threshold That was two years ago (still trying to get dealer to fix problem). In the past week the car shudders-almost feels like it is downshifting and upshifting or like hitting a patch of ice where car slows and then catches and speeds up. We had the code cleared and checked and still only says catalytic converter. Thank you in advance.