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Po420 code thrown at random times

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05 lgt(stock) has thrown a pending po420 code twice in 2 weeks. Tracking it from a bluedriver and when I turn off the car and go to restart it the code is gone...no idea why. So far I have replaced both o2 sensors (denso), plugs (ngk), one coil pack, replaced the MAP to manifold hose and zip tied all hoses, Cleaned throttle body and replaced the clamps.My fuel ltft stays at about 11-14.8 at idle but drops to the 2-3 range while driving. I'm assuming there is a vacuum leak somewhere but I can't seem to find it. I know it's coming so yes I have checked the "t connector" under the intercooler. I have also looked for rusty spots or holes all through the exhaust. Plan on doing a smoke test tomorrow but curious if anyone has any suggestions or know something I might have missed.


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How many miles on the car? (Assuming the catalytic converter has never been replaced). If you are using the OEM TMIC, it could be leaking from the end caps or from the O-ring gasket on the hot side.


If the smoke test is inconclusive I would also check the catalytic converters with an infrared thermometer like relative4 mentioned. Temperature should increase after the cat compared to before by at least 50 degrees. You can find a cheap thermometer at Home Depot for like $20 or sometimes Autozone may have one in stock.

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