Check Engine Light ON

  1. Excessive EGR Flow

  2. Gas Cap

Caused by Excessive EGR Flow (Code P0402) on M2's

by Terry Treiberg -

Higher mileage M2's ('99 and newer) may experience a check engine light due to 'excessive EGR flow'. I recently purchased a '99 model with 70K miles that was having this problem. Fortunately, there is a potential fix for this which can save a bunch of time and money otherwise spent. I say 'potential'
because there are several things that can cause this code. However, many people have tried the following fix with good success. I tried this and it not only solved the problem, but appeared to get my Miata running much better than before. There are several articles in the forum that discuss this, but this will hopefully provide a consolidated article on the issue.

If your check engine light (CEL) comes on, the first step is to have your code read to determine the cause of the CEL (there are a multitude of codes which can cause your CEL to come on). While the Mazda dealer will normally charge a pretty good amount just to read the code ($80 or higher), there are some car parts stores that will read it for free. AutoZone, for example, will read the code for free. Be aware that the OBDC-II connector is inside the fuse box located just down and to the left of the steering wheel. There is another diagnostics connector in the engine compartment that will not fit the OBDC-II scanner tool, and you may have to point the technician to the proper connector (like I did) if you take it to a place like AutoZone.

If the code is P0402, the diagnosis is excessive EGR (exhaust gas recalculation) flow. The EGR system recalculates exhaust back to the intake manifold to lower the oxygen content of the intake gases, thereby cooling the combustion chamber, which in turn lowers NOx emissions. This is done by opening and closing an EGR valve. The Miata's computer calls for the valve to open and close based on a number of factors. If the computer senses that this is not being done properly (valve stuck open or closed or lines plugged), then the CEL is turned on. As it turns out, by the time the Miata has a good number of miles (maybe over 50K or so), the intake manifold can build up an excessive amount of carbon because of the exhaust gas recirculating and the crankcase vapors being drawn in. This causes a small EGR passageway in the intake manifold to become plugged, which eventually causes the light to come on.

Cleaning the little passageway involves removing the top half of the intake manifold. The job is not too involved and can be done without special tools, but will take about an hour or so to do it properly. Some of postings indicate dealers charging anywhere from $150-$750 to perform this service, so you can save some money if you have the time and desire (like I did). It is also a great time to clean the throttle body since we will have it off. Cleaning the throttle body can remarkably improve the way the car idles and will often solve the idle drop problem that many M2 owners have experienced.

Here are the steps:

  1. Remove all of the hoses and electrical connectors that are attached to the top half of the intake manifold and the throttle body. If you are standing in front of the car facing the engine, the manifold/throttle body will be on the left side of the engine. Also, detach the air intake piece that is connected to the throttle body.
  2. Remove all of the bolts from the top part of intake manifold (7 bolts), a couple of small brackets connected to the manifold, and the bolts that secure the throttle body (4 bolts). There are also two bolts below the throttle body that need to be removed. Be sure to remove the coolant hoses
    that connect to the throttle body. You can use a couple of the bolts you removed to plug the hoses so coolant doesn't leak all over the place. Keep track of where all these bolts go so you don't find yourself guessing in the reassembly process.
  3. At this point, you should be able to remove the top half of the manifold and the throttle body. Be very careful with the gasket between the two halves of the manifold, because we should be able to reuse it as long as we are careful. You will probably notice an enormous amount of 'black gunk' on the inside of the manifold and even inside the throttle body around the butterfly valve.
  4. Now begins the cleaning. It is very important to use 'Throttle Body Cleaner' and not carburetor cleaner to perform this cleaning. Other cleaners can damage oxygen sensors, catalytic converters, and can remove the special finish on the inside of the throttle body. You should be able to pick up some 'Throttle Body Cleaner' at an auto parts store. Although the important
    cleaning will be the small hole that I will describe next, you should go ahead and take to the opportunity to clean all of the top manifold using a cloth and your spray cleaner. (I did not clean the lower half because I did not want all this junk running down into the engine, and removing the lower half involves much more work.) Also, clean the throttle body while you are at it. Holding the butterfly valve open, use some of the cleaner and a very soft cloth and swipe the inside until it is clean. Where is this all-important hole? It is located in the inside of the top manifold piece, just an inch or two behind where the throttle body connects to the manifold, on the engine side. The passageway connects this hole to a passageway on the lower manifold. It is a hole about the size of a pencil and you will probably find it nearly plugged with black carbon and oily material. I used
    a plastic tie-wrap plus a bunch of the cleaner spray to get it clean again. Although I didn't have one, a pipe cleaner may be an optimum tool for doing this.
  5. Put everything back together in reverse order. All bolts should be torqued to specification and a little bit of anti-seize compound on the threads is a good idea. Make sure to connect all the hoses and sensor connectors (or else your light will come back on for other reasons!).
  6. Now you need to clear your 'Check Engine' light. There are a couple of ways to do this. The easiest is probably just to pull the BTN1 fuse in the fusebox located under the hood on the passenger side for about 30 seconds. If you pull it out too short, you won't reset the code. If you pull it out too long, you'll lose your radio presets.
  7. Take it for a test drive. You should find your car running as good, and hopefully better than before you started. Not having a CEL right away does not mean you necessarily fixed the problem. It usually takes between 10-50 miles for it to come back on again if the problems persists. If you have gone at least 100 miles since doing the fix wth a CEL, you are probably free and clear! If the code comes back, then there is something else wrong. Good luck!!

Gas Cap

by John Wishart

If you do not tighten your gas cap sufficiently, the Check Engine light may come on. (Experienced on a 2001 Miata SE.) The light will not go out after you tighten the gas cap.

To put out the light:

  1. Insure the gas cap is tight (turn clockwise until it clicks).
  2. Disconnect one of the battery leads.
  3. Step on the brake pedal for 30 seconds.
  4. Reconnect the battery lead.
  5. Start engine and make sure Check Engine light is out.
  6. You will need to reset your radio stations.

Back to the Garage

27 July, 2003