P0420 to P1491 now

Question

Hi! I have 2001 honda prelude. I had p0420 last month then we clean the egr valve and drill the clog on the top side of the egr to clean clogged holes but we didnt open the pan all the way cause its too much work for my friends and then they replace my egr valve and clean the other areas connected to my egr but we never open the pan and we put seafoam. After that after two weeks it came back on today but different code i have p1491 now then my mechanic cleared it away tells me to bring it back next time. But i wonder how come the code changes all of the sudden and i just had timing belt done today and tune up today. My car runs like brand new but mean while i have this code p1491. Any suggestions? I have new egr already. I kept my car maintained.

Answer 1

P1491 Malfunction in the EGR valve system

When it comes to this code and you replaced the EGR system now is the time to check for the vacuum source for the EGR system. If the vacuum source for the EGR is present when it is supposed to be there, then  you have a fault in the EGR position sensor. And for P0420, why the change, once you clear the code the monitor are reset, so this mean you need to meet a drive cycle for the catalytic to be tested. The EGR system is tested first before the catalytic so the CK ENG LT came on. for the EGR system. and the monitor for the catalytic is not ready or not tested