QuestionI have a 97 corolla throwing a check engine light. My OBD scanners read the code as PO401. Insufficient egr flow. I did all the tests in the haynes manual except for the map sensor. Can this be something else besides the map sensor, egr valve, VSV or egr vac modulator?
AnswerThe map sensor has nothing to do with this, for some reason the exhaust gas temp sensor is not seeing enough temperature so the computer turns the check engine light on.
Basically, the egr gas flow is not sufficient, like the scantool says, the most common problems are the egr modulator and/or the vsv.
Does the engine die or run rough when you apply vacuum directly to the EGR valve?
Is there vacuum from both ports on the throttle body when revving the engine?
Is there vacuum at the single port of the modulator when revving the engine?
Is there vacuum at the hose to the EGR valve when revving the engine?
If there is no vacuum signal past the modulator remove it and inspect it, if you can blow through the bottom port it is leaking and needs to be replaced.
If the modulator seems ok I would suspect the EGR VSV.
If everything is working good but there is still a problem there is a possibility that the EGR temp sensor is bad.
