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Q: Lexus shudders while driving. Is catalytic converter the most likely problem? on 1996 Lexus LS400

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
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.
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cat is bad and needs to be replaced. the other issue is most likely the torque converter in the trans. it most likely has to be removed and overhauled.

Roy
Took the Lexus in and the repair person said it was just a loose spark plug. He tightened the plug but we test drove it and symptom was still there. He opened it up again and found that a spark plug wire had damage and was jumping spark to ground. He replaced both caps and rotors and put new plug wires on--said that the plugs looked fine. After putting everything back together he said it made a horrible noise. He went back in and He found that the tensioner jumped the track and put a couple of razor-like cuts in the timing belt. $700 isn't bad for the work but I'm not sure that the work done didn't cause the timing belt problem. He said the water pump looked fine so he didn't replace it. The engine was replaced at 179k with a used Japanese engine by a very reputable shop. The Lexus now has 231,000. We picked up the car and had a 6 hour drive that day. The temp started climbing, a symptom we had before, but stayed hotter for a while even when driving faster. It settled down on the freeway though but now we didn't have the heater fan work properly. By the time we made it back home the temp panel controls were hot to the touch and still not warm enough in the car. 10 minutes before home the outside temperature gauge changed from the correct 41 degrees to minus 22 degrees.
Frustrated and not prepared to spend more right now.
DO NOT BUY A CONVERTER U WILL BE WASTING MONEY........SCAN TOOLS ARE HELPFUL BUT WRONG ALOT OF THE TIME.....YOU HAVE A AIR LEAK.......AND THE AIR LEAK IS OVERLOOKED BY PROFESSIONALS BECAUSE THEY ARE GONNA RIP YOU OFF....I HAD SAME PROBELM.....IT COST ME 79CENTS TO FIX,,,SAD HOW PEOPLE ARE SO DISHONEST
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