Hit 100,000 miles in my rusty F-150 and the engine light finally made sense

I rolled over 100,000 on the way to a job in Akron this morning, and right at the same moment the check engine light came on steady instead of blinking. I pulled over, grabbed my $25 code reader from Amazon, and it threw a P0420, which I've seen three times before but always ignored. Turns out the O2 sensor was reading lazy because my rear cat is clogged with carbon, not because the truck is dying. For a decade I thought that code meant the whole exhaust was toast, but a simple backpressure test with a vacuum gauge showed it's just one part. Has anyone else run a P0420 for years and then found out it was the easiest fix they put off?

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kellygrant4d ago

Man that vacuum gauge trick is the kind of thing you wish you knew ten years ago. Sounds like you basically saved yourself a thousand bucks at a shop just by doing one simple test instead of throwing parts at it. The annoying part is most code readers just tell you the code, not what to actually check first, so people end up replacing sensors and converters that were fine. Now that you know it's just the rear cat, that's a weekend job with a sawzall and a wrench if you're handy, or even a cheap universal replacement if you're not. Wish more folks would post stuff like this instead of just saying "it's the cat, replace everything.