Given the high mileage, I recommend performing a compression test to rule out cylinder 3 issues before considering further repairs. The P0303 code indicates a misfire in cylinder 3, which could stem from worn piston rings, valve problems, or low compression—common in vehicles with over 300,000 miles. Even though the car starts reliably and runs well under normal conditions, the worsening idle when heating or using A/C, and misfiring during uphill driving or at stoplights suggest a mechanical issue in that cylinder. Replacing spark plugs and fuel injectors has been done, but these components may not resolve underlying engine problems. A compression test is essential to confirm whether the misfire is due to internal engine wear or external ignition/fuel system faults.
Given the high mileage, I recommend performing a compression test to rule out cylinder 3 issues before considering further repairs. The P0303 code indicates a misfire in cylinder 3, which could stem from worn piston rings, valve problems, or low compression—common in vehicles with over 300,000 miles. Even though the car starts reliably and runs well under normal conditions, the worsening idle when heating or using A/C, and misfiring during uphill driving or at stoplights suggest a mechanical issue in that cylinder. Replacing spark plugs and fuel injectors has been done, but these components may not resolve underlying engine problems. A compression test is essential to confirm whether the misfire is due to internal engine wear or external ignition/fuel system faults.