PO 401 code - 3.4 liter - where else to look?

1995 T100 3.4 liter auto trans. 326,000 miles

Last week the Check Engine Light came on. Upen scanning a PO401 code showed up, indicating insufficient EGR flow. Pulled the EGR, not plugged with carbon. Nor are the inlet pipe to it or the manifold that it bolts onto. With vacuum applied, the pintle moves up as it should. Reinstalled the EGR. Pulled the cap off the EGR vacuum modulator and cleaned the filter. Reinstalled. Pulled the fuel injection fuse to reset the light. It came back on after about 20 miles. So now, the possibilities are the EGR temp sensor and the EGR vacuum switching valve. Unfortunately, the Haynes manual I have doesn't have parameters or test procedures for either one. I know the temp sensor detects temperature rise to tell the ECM that the EGR is passing exhaust gas in sufficient quantity. Does anyone have experience in this area that would like to share?

Thank You

I am having a similar problem with my truck, I am getting the P0402 code. I found this information on the net. It may help you out. My light comes on every 200 or so miles, or when my wife drives my truck. You said you cleaned the filter on you EGR valve, is that easy to remove?

DTC P0402: EXCESSIVE EXHAUST GAS RECIRCULATION (EGR) FLOW DETECTED

CAUTION: If ECM replacement is instructed in following testing, and ground circuit are okay. If either are suspect, malfunction.

Circuit Description

The EGR system recirculates exhaust gas through intake system to EGR valve opening is regulated by the EGR Vacuum Switching Valve VSV is controlled by the Vacuum Control Valve (VCV). Amount of EGR which is mounted on EGR valve.

The EGR valve is closed (VSV on) under the following conditions:

Engine is not warmed up. Vehicle under deceleration. Light engine load. Engine speed more than 4000 RPM. Engine idling. Code is set when EGR cut-off, lift amount of EGR valve is .1" (2.6

EGR valve stuck open. EGR Vacuum Switching Valve (VSV). EGR VSV open circuit. EGR valve position sensor open or short circuit. EGR valve position sensor. ECM. Test Drive Confirmation

1.Connect scan tool. See Fig. 1. Start and warm engine to normal minutes or more. Stop vehicle and allow to idle for about 2 2.Start engine and allow to idle. Operate vehicle at 43-56 MPH minutes. 3.Check READINESS TESTS. If COMPL is displayed on scan tool and displayed and MIL is not illuminated, repeat test drive

Diagnosis & Repair (Using Toyota Scan Tool)

1.Check all EGR system vacuum hoses. See appropriate hoses as necessary. If all hoses are okay, check EGR VSV. VSV when activated by scan tool. VSV is located under "V" bank Go to next step. 2.With EGR system off, air from port "E" should flow from port flow from port "F". If VSV operates as specified, go to step 3.Remove EGR VSV. Using ohmmeter, measure resistance between VSV (20° C). Measure resistance between each terminal and body of specified, replace VSV. If resistance is as specified, go to 4.Check VSV operation. Apply battery voltage to VSV terminals. "F". Remove battery voltage from VSV terminals. Air should specifications in steps 3) and 4), check for a short in EGR diagram in L - WIRING DIAGRAMS article. Repair wiring as 5.Disconnect vacuum hose from EGR valve. Connect a vacuum pump

If engine does not run rough or stall, replace EGR valve. If engine runs rough or stalls, check EGR valve position sensor. See appropriate I - SYSTEM/COMPONENT TESTS article. Replace position sensor as necessary. If position sensor is okay, replace ECM.

Diagnosis & Repair (Using OBD-II Scan Tool)

1.Check all EGR system vacuum hoses. Replace hoses as necessary. If vacuum hoses are okay, access ECM behind glove box. Ensure ignition is off. On models with Calif. emissions and except Calif. emissions with engine immobilizer system and/or traction control, disconnect ECM E11 connector. See Fig. 2. Connect a jumper wire between ground and terminal No. 18 (Yellow/Green wire) at ECM E11 connector. Go to next step. On except Calif. emission models without engine immobilizer system and/or traction control, disconnect ECM E8 connector. See Fig. 3. Connect a jumper wire between ground and terminal No. 12 (Yellow/Green wire) at ECM E8 connector. Go to next step.

NOTE: For information on engine immobilizer system, see COMPUTERIZED ENGINE CONTROLS in E - THEORY/OPERATION article.

2.On all models, apply air pressure to EGR Vacuum Switching Valve (VSV) port "E". VSV is located under "V" bank cover on top of engine. With jumper wire connected, air should flow from port "F". See Fig. 6. With jumper wire disconnected, air should flow from port "G". If VSV operates as specified, go to step 5). If VSV does not operate as specified, go to next step. 3.Remove EGR VSV. Using ohmmeter, measure resistance between VSV terminals. Resistance should be 27-33 ohms at 68° F (20° C). Measure resistance between each terminal and body of VSV. Resistance should be infinite. If resistance is not as specified, replace VSV. If resistance is as specified, go to next step. 4.Check VSV operation. Apply battery voltage to VSV terminals. Apply air pressure to VSV port "E". Air should flow from port "F". See Fig. 6. Remove battery voltage from VSV terminals. Air should flow from port "G". Replace VSV as necessary. If VSV meets all specifications in steps 3) and 4), check for short in EGR VSV circuits between engine compartment fuse box and ECM. See appropriate wiring diagram in L - WIRING DIAGRAMS article. Repair wiring as necessary. 5.Disconnect vacuum hose from EGR valve. Connect a vacuum pump to EGR valve. Start engine. Apply vacuum to EGR valve. If engine does not run rough or stall, replace EGR valve. If engine runs rough or stalls, check EGR valve position sensor. See appropriate I - SYSTEM/COMPONENT TESTS article. Replace as necessary. If EGR position sensor is okay, replace ECM.