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1999 Blazer 4.3L Engine P0401 and P0405: Diagnosis and Solutions

Model: 99 blazer 4.3 Fault Code: P0401 Posted: 2015-11-13 08:00

I own a 1999 Blazer with a 4.3L engine that is throwing trouble codes P0401 and P0406. The vehicle has accumulated 93,000 miles. I have already replaced all components in the evaporative emission (EVAP) system and inspected and replaced all visible vacuum hoses. Additionally, I've replaced the EGR valve. Despite these efforts, the codes persist. Could there be other underlying issues I'm missing? What are common causes of P0401 and P0406 on this model?

Related fault codes

P0401P0406

Comments (2)

Anonymous 2015-11-13 09:35

Verify the exact trouble codes using a reliable OBD2 scanner. Ensure you're reading both P0401 and P0406 correctly, as these codes can indicate different issues. Confirm that your scan tool is properly calibrated for the 1999 Blazer 4.3L engine.

Anonymous 2015-11-13 11:08

P0401 typically indicates insufficient EGR flow, which may stem from clogged EGR intake passages or a faulty EGR valve. However, there is no official P0406 code for the 1999 Blazer—this might be a misread or incorrect code. Avoid replacing EVAP system components solely based on codes without proper diagnostics. Use a bi-directional scan tool to manually energize the EGR solenoid and listen for engine roughness, RPM drop, or stalling to assess passage cleanliness. A bad EGR valve or wiring could also be the cause—circuit testing is recommended before replacement.