P0404 OBD2 Code on 1998 Chevrolet Blazer: EGR System Failure Diagnosis and Fixes
I recently received a P0404 diagnostic code from my 1998 Chevrolet Blazer, which indicates an 'EGR system failure' — specifically, either an open or short circuit in the EGR position sensor or a malfunctioning EGR valve. I replaced the EGR valve as recommended, but the vehicle still stalls at low speeds (below 10 mph) and runs poorly under load. When I temporarily disconnected the EGR position sensor, the engine ran smoothly without any stalling. This suggests that either the new EGR valve is defective or there's an issue with how the system is being commanded. However, since the sensor itself is brand-new, I'm concerned about whether the replacement part was faulty. I understand that a P0404 code means the actual EGR position doesn't match the commanded position — this requires a scan tool to verify real-time valve operation and proper communication between the engine control unit (ECU) and the EGR system. I'm now looking for guidance on how to properly diagnose this issue without relying solely on a professional shop, especially given the cost of diagnostic tools and their learning curve.
Please avoid using all caps — it's not necessary and can come across as aggressive. A P0404 code does not automatically mean you should replace a part. It indicates that the actual EGR position sensor reading differs from the commanded position by more than specifications. To properly diagnose this, a scan tool is required to command the EGR valve open and closed while monitoring actual position in real time. Additionally, check for wiring faults such as opens or shorts in the EGR circuit.