Chevy Uplander P1174 Fault Code: Causes and Diagnosis Tips

I'm experiencing GM-specific fault code P1174 on my Chevy Uplander. I've taken it to three different garages, but they seem to be guessing at the root cause. I've already had injectors cleaned and a faulty spark plug replaced. Additionally, both oxygen sensors have been replaced. Despite these repairs, the P1174 code persists. The code indicates an issue with the oxygen sensor switching behavior, suggesting a fuel trim imbalance between cylinders. I'm looking for reliable information on what could be causing this specific fault code in my Chevy Uplander model.