I think I have a pretty good line on this, but wanted to post the process.

Last week, I got the idiot light, checked and found OBD code PO402. Reset, and everything was fine.

Last night, I got the idiot light again, with terrible fuel mileage (going over Donner Pass, so long uphill pulls). Mileage was so bad I stopped to check for a fuel leak! I had the OBD reader with me, found PO401, reset, and got it again about 10 miles later

This combination pretty much convicts the DPFE sensor. Today I will check the DPFE sensor and EGR valve, clean the lines with Seafoam, and toss the rest of the can of Seafoam into the tank.

If the problem returns, I'll add to this.

Here, BTW, is a test procedure for checking DPFE and EGR:


Testing DPFE Sensor and EGR Valve

SYMPTOMS:

OBDII codes PO401 and/or PO402, poor fuel mileage.

Note: A disconnected Throttle Position Sensor may also give code PO401

TOOLS:

Voltmeter w/ positive (red) probe sharp and narrow enough to make contact inside the electrical

connector attached to the DPFE sensor (a pin can be used for this)

Vacuum tester (such as Minivac)

Hose(s) to connect pump to ports on DPFE sensor and EGR valve


PROCEDURES:

Testing DPFE Sensor:

1) Connect the voltmeter probes, negative (-) (black) to ground and positive (+) (red) to the

top lead in the electrical connector (do not disconnect the connector). This is the active

feedback signal lead.

2) Disconnect the two hoses from ports on the sensor, and connect test hose between the vacuum

tester and the port closest to the front of the sensor module.

3) With the ignition switch in the "ON" (running) position (engine does not need to be

running), voltmeter should show approximately 0 volts DC.

4) Increase vacuum with the tester. The voltmeter should show an increase, up to approximately

5 volts DC.

5) If voltage does not increase, or does not vary with vacuum changes, DPFE sensor has failed

and must be replaced.

6) Repeat steps 2 - 5, except connect to the rear (reference) vacuum port. Voltage will be

significantly lower, but should vary with vacuum changes.

7) If sensor is operating, reconnect vacuum hoses.

8) Test EGR valve.



Testing EGR Valve:

1) With voltmeter test probes still in place (see DPFE sensor test Step 1, above), disconnect

hose from port on EGR valve, and connect test hose between EGR valve port and vacuum pump.

2) Start engine.

3) Apply slight vacuum to the EGR valve (this will open valve). Engine may stumble or almost

stall as EGR valve opens; this is normal, and indicates proper operation of valve. Voltmeter

connected to DPFE Sensor should show increase in voltage, as during sensor testing, above.

4) If valve fails to operate, clean and test again, or replace valve and gasket.

 

I replaced the DPFE module with an Auto Zone lifetime warranty unit.

The codes went away, but mileage still bad.

I disconnected the battery for an hour or so, mileage still bad.

I replaced the fuel filter and PCV valve. Mileage SEEMS to have recovered. If I have to do anything else, I'll add another update. If I don't post again in this thread, it's all back to normal.