2003 Chevy Express 3500 P0154 and P0300 Codes: Check Engine Light at 75 mph+
Hi, I'm new to this forum. My 2003 Chevrolet Express 3500 has been showing a check engine light that activates above 75 mph. The van runs smoothly under normal conditions. - OBD2 Diagnostic Codes: P0300 (Random/Multiple Cylinder Misfire) and P0154 (Oxygen Sensor Circuit Malfunction) - Issue triggers only when speed exceeds 75 mph - Mileage: 92,xxx miles - Spark plugs and wires are original installation - No exhaust smell detected - No rough idling or stumbles during operation I understand that P0154 indicates a lack of signal from the oxygen sensor. What should I check first? I'm planning to replace the spark plugs and wires anyway, but I'd like to prioritize based on likely causes. Additionally, I've noticed a whining noise coming from the alternator—especially after the vehicle has sat for several days. This could be a sign of failing bearings inside the alternator?
The whining sound is likely due to worn alternator bearings, but it could also stem from a loose belt tensioner, idler pulley, or water pump. Regarding the P0154 code, replacing the oxygen sensor with a new one from your local Chevrolet dealer is recommended. Additionally, spark plug and wire replacement—along with a full tune-up—is overdue for this vehicle model.