Where is the low side valve for AC on a 2003 Chevy S10 4 cylinder?

The low-side service port (where you'd connect gauges to check refrigerant pressure) on a 2003 Chevy S10 4-cylinder's A/C system is typically located on the accumulator/dryer. The accumulator/dryer is usually near the front of the vehicle, close to the firewall, and often fairly low to the ground. It's a cylindrical canister, sometimes with a sight glass.

However, exact location can vary slightly depending on specific model and trim level. You'll need to look closely in the area mentioned above. It is highly recommended you consult a repair manual specific to your 2003 Chevy S10 4-cylinder. These manuals will have diagrams showing the exact location and other crucial details about your A/C system. Working on an AC system also involves handling refrigerants that require special care and handling equipment.