Help! I just bought a used 2001 Chevy Silverado. I had it for about three weeks when the check engine light came on. I had to have a sensor replaced (forgive me- I know very little about cars, but according to the paper work a “knock sensor” was replaced). Drove for about three weeks with no problem. Today, the light is back on. I’m just so frustrated because I spent $476.00 not a month ago and I’m having another issue. Eek! Any advice?
You need to have the codes read again.
Some parts stores like Auto Zone do this for free.
Tester
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Yep. Or you can buy your own code reader off Amazon, a decent one will cost about what it takes to fill that truck up. Try this one: https://smile.amazon.com/Automotive-Diagnostic-Diagnostics-Scanners-FOXWELL/dp/B01N6RKE29/
In all likelihood you’ve got a different sensor going bad, e.g. an oxygen sensor in your exhaust (note, I have no idea whatsoever what this vehicle has). But it could be something more serious, e.g. needing a new catalytic converter. So get them checked.
What engine?
4.3?
4.8?
5.3?
6.0?
If you have the 4.8, 5.3 or the 6.0 . . . did you replace both knock sensors AND the knock sensor subharness?
It’s very important that you did, because the knock sensors are known to severely corrode, and the subharness gets extremely brittle. If you take off the manifold, it makes sense to do replace all those parts I mentioned
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Ok so the service place called and said it’s a catalytic converter code. He said it’s avout a $1,200 repair and I’m sick over it. He also said they cleared the code and drove it around and it did not come back on so if I want to wait on the repair I can. I think he was just trying to be nice since I just bought it and thy just did a repair. So, should I wait? Do the service? Find another service place? Thanks so much! I’m a farm girl, just not a mechanical one 
The light is off now so use the time to call independent muffler shops for an after market converter price. .
I’d give this a try before spending $1,200 on replacing the cat.
Tester
You might also ask them which specific code they read. It might not be the catalytic converter causing the code to show up…
Not all “cat codes” mean it a bad cat. Please post the actual code here, i.e. P0420. If the place that told you it was a cat code won’t give you the actual code, then go to a place that will read the code for free. If you use AutoZone, ask for the printout from their cash register where they put in the code, it will be at the top.
You need more details
Did the guy drive the truck long enough and under the right conditions, so that the cat readiness monitor ran to completion?
If he didn’t, then his test drive wasn’t very productive . . .
If that particular P0420 or P0430 code does return and a bad cat is definitively diagnosed . . . you should consider going to a reputable exhaust shop and having a quality cat welded in. It’ll be a lot less than $1200
Well, he didn’t drive it long enough because the light is on again this morning. 
I’m going to take it to auto zone and get the exact code. I’ll post it here when I know it and would appreciate your advice/direction. I know so little about cars, I just want to get this truck fixed and caught up on maintenance so I don’t keep putting money into it every three weeks.