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Dan, Technician
Category: Car
Satisfied Customers: 23473
Experience:  Independent Shop Experience, 2 Year Automotive Diploma, & Access to Information Database
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I have a 2002 Mazda 626 2.0 4 Cyl, the car runs fine but I

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I have a 2002 Mazda 626 2.0 4 Cyl, the car runs fine but I need to get the check engine light off for inspection time. The code is P0400, I have ensured there are no exhaust leaks, the front and rear cats have been replaced along with the EGR valve. The car has also just had a tuneup consiting on new fuel filter, pcv valve, oil, spark plug and wires. I have also ran RXP through the gas about a month ago and when changin the EGR I took off the throttle body and cleaned inside and sprayed air through the holes to ensure they were no clogged. Any suggestions?

graphicThis problem can only be related to the egr valve. You can try to do a resistance test on all wires between egr valve and pcm. You can also make sure no vacuum lines are clogged or leaking, this is usually caused by clogged egr passages so you may want to make sure they are clean and use a metal coat hanger to clean along with cleaner, the problem could also be the boost sensor vacuum solenoid, The boost sensor should read about 4.5 volts with no vacuum and about 1.3 volts with vacuum, improperlly installed egr gasket could cause this problem. Here is a code definition and location picture of boost solenoid and boost sensor. If you cannot find problem with this information you may need to bring vehicle to shop with high end scan tool so they can moniotor pcm operation or calibrate needed sensors.




Control System Component Location

Testing and Inspection


  1. Remove the EGR boost solenoid valve.

  1. Inspect airflow between each port under the following conditions.
    • If not as specified, replace the EGR boost solenoid valve.

P0400 EGR System Flow Malfunction

Possible CausesSetting Conditions
  • EGR boost solenoid control circuit is open or shorted to ground
  • EGR boost solenoid power circuit is open to the Main Relay
  • EGR boost solenoid has failed
  • EGR boost sensor has failed
  • EGR valve assembly is leaking, damaged or has failed
  • EGR valve vacuum hose(s) loose, damaged or disconnected
  • MAF sensor or TP sensor has drifted out-of-calibration
  • Exhaust pipe to EGR boost sensor or solenoid leaking/plugged
  • PCM has failed

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