2004 Chevy Trailblazer LT Fault Codes P0016, P0495, and P1860: Causes and Fixes

I own a 2004 Chevrolet Trailblazer LT with an inline 6 engine and 4-wheel drive. My vehicle is currently displaying three diagnostic trouble codes: P0016, P0495, and P1860. I've been taking it to garages repeatedly, but they often can't identify the root cause—perhaps because I'm a woman or maybe due to my lack of technical knowledge. I want to stop being ignorant about these issues. The car is now refusing to shift past second gear, which significantly impacts driving experience and fuel efficiency. Additionally, several instrument panel gauges have become erratic: the temperature gauge frequently reads beyond the maximum scale or fluctuates around mid-range; the gas gauge often shows 'full' even after refueling, sometimes reading over full halfway through a fill-up; and the battery gauge occasionally spikes to 100% before settling back to normal. I've already replaced the thermostat, but these symptoms persist. I'm seeking reliable information on what repairs or diagnostics are needed for these specific codes in my 2004 Chevy Trailblazer LT model.