I have a 2001 Subau Outback, 2.5 litre, standard transmission. At 40 miles per hour, shifting correctly, the engine will start to surge. Immediately after it surges there is a "tock" noise just like you get when you stick your tongue on the roof of your mouth and quickly remove it., The "tock" noise sounds like it is coming from under the passengers side front door. The car threw a Po 420 code a couple of days ago. I have not had the chance to check the cat or replace the O2 sensor. Could you give me some insight as to whether the code, the surging, and the noise could be related? Thanks.
SOURCE: 2000 Subaru Outback Wagon (100K miles) with P0420 Code
I do not know for sure if this is the problem you are having, But, I have noticed that from 95 up to 2002 I have seen a lot of PO420 codes that have been just the sensor. I do not know if it was a weak sensor or just that program telling it that it is below threshold. I did notice that if the vehicles are run for a long trip at highway speed it goes out. I wish you luck and have a great day.
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