If the EGR valve is working I’d guess the problem is the VSM, its electrical connections, or the vacuum hoses. As other output sensors fail, the fault codes will accumulate in the PCM until the problem has been identified, fixed and the fault code(s) erased. Where I’d start w/that problem is connect a hand-held vacuum pump to the EGR vacuum input, warm the engine to normal operating temperature, and see if applying vacuum to the valve turns it on and stalls the engine or not. So if a Calif car, that part has to be working too.
This measurement is also used by the computer to verify the EGR recirculation is correct. The Calif EGR contains a thermistor to measure the EGR’s temperature. One other idea, if this is a Calif car (like mine), it probably has a special Calif EGR. Manifold vacuum is incorrect, but this would usually create other very noticeable problems.Device that measures manifold vacuum is fault (MAP).Problem w/ vacuum hoses between throttle body and modulator and EGR.So the problem pretty much has to be one of these The EGR modulator has vacuum hose inputs from the throttle body, and a vacuum hose output to the EGR valve. When the VSM is commanded on, the amount of EGR to actually apply is determined by another gadget, the EGR modulator. The computer’s control of the EGR is via an electric-selenoid operated vacuum switching valve (VSM). The P0304 code will be seen in cars where the ECU has detected that cylinder 4 is not firing properly. This code may be seen with other misfire codes and lean / rich codes. My Corolla’s EGR configuation is pretty much identical to the 4-banger version fro the 98 Camry P0304 is a diagnostic trouble code seen when a misfire is detected in cylinder 4.
On your engine the vacuum reading isn’t going down sufficiently to please the computer for some reason. When the EGR turns on, that’s sort of like a vacuum leak, so the intake manifold vacuum should diminish a little. P0401 (for the 4 banger engine) is saying the computer is commanding the EGR valve on, but when it measures the intake manifold vacuum, the vacuum reading there isn’t consistent with the EGR being on.
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