If the check engine light is on in a 1996-2003 1.8L Mazda Miata, and/or DTC codes P0400 and P0402 are set, it is likely that the exhaust gas recirculation (EGR) passages in the intake manifold are restricted with carbon.
The EGR passages are located in the intake manifold behind the throttle body. To clean the EGR passages, the throttle body must be removed. Depending on the model, there can be one passage or two. The passages can carbon up so completely that it may be necessary to scrape inside the manifold to locate the passages.
Once the passages are located, clean all the carbon from them. Remove the EGR valve and verify there is no carbon in the valve as well.
Then locate the EGR boost sensor using a service manual or underhood label and verify the vacuum hose going to the boost sensor is clean and installed correctly. The boost sensor is used to monitor EGR flow. Install the throttle body and clean the code, then test-drive the vehicle to verify the EGR monitor passes and the code does not reset.
Courtesy of Identifix.
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