P0300 and P0128 Fault Codes - 2002 GMC Silverado 4.3L V6 Diagnosis
The vehicle is displaying fault codes P0300 (Random Misfire) and P0128 (Coolant Temperature Sensor Circuit Malfunction). For the random misfire (P0300), inspect spark plugs for signs of wet, black, dry, or tan deposits. Only AC Delco plugs are recommended for this engine. Ensure all cylinders are firing properly. Check fuel pressure using a gauge under KOEO (Key Off Engine Off) conditions and monitor for any drop in pressure over five minutes. For P0128, verify that the thermostat is present and rated at 195°F. A faulty thermostat may cause improper engine warm-up and contribute to misfiring. To test thermostat functionality, remove it and submerge in a pan with a temperature probe to confirm proper opening behavior. Alternatively, run the engine and monitor coolant temperature readings to observe if the thermostat opens as expected.
Yes, the fault codes are P0300 and P0128. First, check the thermostat—ensure it is a 195°F model. Use an OBD2 scanner to verify the coolant temperature sensor reading; inaccurate readings can lead to fuel delivery issues. For misfire symptoms, inspect spark plugs for signs of wet (black), dry (tan), or inconsistent combustion. Only AC Delco plugs are recommended for this engine. Confirm all cylinders are firing correctly. Also, check fuel pressure with a gauge under KOEO conditions and observe any drop in pressure over five minutes.