ECU code help please

[ziya20]

As I try to read my ECu codes, I have 5 long and 9 short blinking plus 3 long and 4 fast one after that.

I know 0304 is a knock sensor code but what is 0509 help please.

[THT]

What year is your car? If it's a 4th gen, 0509 is the left bank, upstream O2 sensor (high voltage fault). It's the one closer to your bumper.

~THT

[ziya20]

that is 1998.
Is that first o2 sensor going from muffler toward front.

If yes it is bizzarre, cause I gues I changed that one 3 months ago.

Now what do you recommend, should I change knock sensor first, or o2 sensor.

Thank you for your help.

[THT]

Always always always change the KS last. 4/5 times, the O2 will trip the KS code. And the one you need to change is toward the front of your car, in the first portion of the downpipe.

~THT