HELP! Car doesn't pass smog check. Now knock sensor went bad
 

96 GXE Max with 93K miles
My car failed the NO test (max 430. I got 470). The mechanic claimed to have cleaned the EGR, but the levels are still too high. He then tells me that he will replace a small valve with a bigger one, so the NO levels can go down during the test(from the EGR to the cylinder block I guess). Since the ECU got resetted he tells me to drive the car around for a few days, so the ECU can relearn everything.
The next day I get a check engine light!! I take it back to the mechinic and the code is about a knock sensor error. Now, I know the knock sensor doesn't throw a CEL. After he resetted the ECU I checked the next day (no CEL) and indeed I got a knock sensor code.
This mechinic is an honest guy, but I guess I overestimated his skills. Did my knock sensor went bad because of the little (~5 inches long) valve change?

What could the problem be? It can't be the knock sensor since the first time I failed it was obviously working. Also, what could have triggered the first CEL?