Oxygen sensor/antiknock sensor

2001 GMC 2500 has "service engine" in message center
local oil change shop says error messages indicate problem with O2 sensor and anti-knock sensor. They recommend replacing all 4 O2 sensors but told me to take truck to someone who works on engines to fix antiknock sensor. Their part will cost $569. And said other sensor replacement may be double that. I will do it if necessary. What do you recommend?