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EGR related hoses, hard pipes and passageways tend to get clogged with soot so check all of them. The EGR system usually relies on a EGR temp sensor that tells the ECU that hot exhaust gasses are in fact reaching their destination, which is usually the intake for a second pass through the combustion. This lowers NO & CO2 for cleaner tail pipe emission.
If your temp sensor isn't achieving the setpoint, then the ECU knows this and will not set the flag, which I think is happening in your case. One of the cause of the buildup of soot is that the EGR valve isn't opening up when it should so the passageways might be clogged with soot. This could be due to the vac modulator valve that operates the EGR valve isn't doing its job. So, check all that.
Your vac hose routing under the hood will show you all this. This is tedious but well worth the effort. Blow through each hoses and passageways to make sure they're clear. Often times you have to clean out passageways with a thick copper wire, like a #12 solid copper ground wire made for a house. Some folks use a tiny drill bit to clean out short clogged passageways then blow out with an air comp. The FSM has detailed troubleshooting steps for modulators and VSVs as well as the temp sensor.
Hi Ali. Thanks for the detailed reply. I'm going to have to do all of that because I replaced the 3 components but the P0401 came back. I did not replace the temp sensor so I will look into that. I see a lot of rubber tubes that go into the throttle body. I read that I have to take all of that off and clean the passage ways. Obviously you don't want to go shoving soot buildup into there and jam it up even worse. I've attached the vacuum diagram under the hood. No temp sensor on there. I'll keep looking.
To make things even more chaotic, I have a wiring harness plug down by the fuel injectors and it has bare wires coming off of it with two twisted together. It could be something, or it could be a plug that was dropped down in there and never recovered. It could be a s***ty attempt by the last owner to relocate the VSV from the back of the engine to where it is now on the throttle body. I won't know until I get under the car and I need some ramps.
Lastly, I haven't been able to locate a FSM for this car. The best pdf manuals I can find are here: CelicaTech - powered by vBulletin - http://bgbonline.celicatech.com/#6th
I've gone through the service tech manual when looking for how to replace the EGR. It's all text format and where I found the procedure for removing the intake manifold and EGR to get to the fuel injectors. I need torque specs and diagrams but I'm not sure I'm looking in the right place. Please pardon this being amateur hour.
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