O2 sensor codes

Hi everyone,

I have a 2001 Grand Vitara and the check engine light came on. When I checked with a code reader it showed 8 codes (one for each of the four O2 sensors and an additional four of the same number codes, but indicated as pending codes). I was able to reset/erase all of the codes with the exception of the two upstream sensors.

So I replaced the two upstream sensors and when I stared the engine the check engine light was out, but at the next start it came on. When I checked the codes I got the same result as when I first checked it, i.e. eight codes. This time I was able to erase all of them, but at the next start the check engine light again came on.

Is there a time period needed for the codes to be able to properly reset, such as a number of starts? If not, where do I go from here?

Any help would be greatly appreciated.

Tony