Generally speaking you'll know that the EGR modulator needs to be replaced because either the top cap is melted/missing and/or you get the PO401 and just replacing vacuum hose leaks didn't solve the issue. Replacing the modulator is a good time to replace the 3.5mm vacuum hoses. Its been noticed that on turbo or supercharged applications its especially good to keep on eye on the modulator to see if it gets melted from exhaust gas.
INSPECT EGR VACUUM MODULATOR OPERATION
(a) Block ports P and R with your finger.
(b) Blow air into port Q, and check that the air passes through to the air filter side freely.
(c) Start the engine, and maintain speed at 2,500 rpm.
(d) Repeat the above test. Check that there is a strong resistance to air flow. If operation is not as specified, replace the EGR vacuum modulator.
RECONNECT VACUUM HOSES TO EGR VACUUM MODULATOR
(a) Connect the vacuum hose to R port.
(b) Connect the vacuum hose to Q port.
(c) Connect the vacuum hose to P port.
P and R are the ports right next to each other. Q is the single port that goes to the egr.
• (1) 25870-66011 EGR modulator (blue cap)
• (1) 25695-66010 EGR modulator hose
MADE in JAPAN