Tech Feature: Mazda Driveability Diagnostics and Check Engine Light Complaints

Tech Feature: Mazda Driveability Diagnostics and Check Engine Light Complaints

From a torn air intake hose to ignition system concerns, Bob Dowie tackles driveability issues as well as some of the more common ailments that will bring Mazda vehicles into your bays with a check engine lamp complaint.

In this article, we’re going to look at Mazda driveability issues as well as some of the more common ailments that will bring a car into the bay with a check engine lamp complaint. The good news is that these problems are really not all that common and, more often than not, you’ll find yourself doing profitable preventive maintenance. But, remember, it’s being able to fix the tough problem that gets us those more profitable jobs.

The most common failure we see on Mazda models is a torn air intake hose between the airflow sensor and the throttle body. The symptoms that result from this failure are hard to miss. The usual complaint is a big stumble or stall on initial acceleration. As the engine torques on the mounts, the crack in the hose will open, allowing unmetered air into the manifold, while at the same time reducing airflow through the sensor. Of course, when the engine stumbles, the crack closes, allowing the engine to accelerate, starting the entire process over again. Many times, the customer will mistakenly think the transmission is the problem.

Although it’s simple to diagnose with a visual inspection, I’ve seen some good technicians tricked by this one. These hoses become hard and brittle over time, and you may not find the fault simply by squeezing the hose. Be sure to inspect the hoses closely for cracks that won’t be obvious but will become evident when the engine is loaded against the mounts.

Another check engine lamp issue that can give you fits is P0400, excessive EGR flow.

The first problem is the fact that an “excessive flow” code will be thrown, when the actual problem is “insufficient flow.” The excessive report is the result of the ECU commanding more EGR when the engine doesn’t respond as expected to the EGR valve being opened. The most common cause for this lack of response is plugged EGR passages in the manifold. By removing the throttle body and EGR valve, the passages can be cleaned using a liberal amount of carbon cleaner and a piece of speedometer or hand brake cable and a drill.

On some models, it will be necessary to remove the upper plenum. Be sure the vacuum line and manifold nipple are clear going into the MAP-type sensor that serves as the EGR boost sensor. Don’t let the term boost sensor confuse you; it’s a manifold pressure sensor that’s used to monitor the pressure change when the EGR is commanded open. There are service bulletins on the EGR system, and it’s well covered on the iATN website. I would recommend you look at both before getting too deep into the diagnostic process.

On the later models that use an electrically controlled EGR, there have been instances where the valve has caused issues with a throttle tip-in stumble, stalling or rough idle. These complaints may be accom-panied by a system lean code. The rough idle is easy to diagnose: Just let the engine warm up and look for flow through the valve by checking to see if it’s excessively warm. We know there shouldn’t be any flow through the valve at idle high temp; if so, it indicates that the spring-loaded valve is not seating.
The stalling issue can be a bit more challenging, since it could be an intermittent problem with the valve not closing. In this case, we check other possibilities for stalling; if we find no problems there, we go with our experience and replace the valve. The same scenario applies with the tip-in stumble: In this case, the weak return spring in the valve is allowing more flow than the ECU is commanding. If you suspect this problem, a road test with the valve disconnected will quickly confirm it.

While a sticking EGR can cause a system lean code, it’s not the most common cause. The first step when faced with a lean code is to be sure to look at the freeze-frame data. Look at the fuel trims and the conditions when the code was set. You’ll probably find a long trim number near 25. This is the result of the short-term trim adding fuel and driving up the long-term number. If you’re looking at a multi-bank engine, take note if both banks are showing a similar number or if it’s just one bank.

Freeze frame will also tell you when the limit was reached. Armed with this information, you can make some decisions. If the limit was reached at idle, it would have you looking for unmetered air finding its way into the engine; it could be the EGR valve, but could also be an intake manifold gasket leak, or the O-ring seals on the V6.

On the four-cylinder, a leaking intake gasket will usually set a misfire code, along with the system lean letting you know where to concentrate your inspection. Usually all it takes is a shot of intake cleaner at the manifold flange and stethoscope to confirm the diagnosis. The six-cylinder’s upper manifold O-ring leaks can be a bit more challenging, and if you’re in doubt, a smoke test will confirm you’re on the right track.

On the other hand, if the freeze-frame information is telling you that the code was set at speed, the idle is smooth and the driver reports no additional symptoms, you would have to think about what makes it lean under those conditions. My first thought would be the mass air sensor miss is reporting the air flow; by now we should all be aware of how debris finds its way to the hot wire sensor.

If you’re going on an initial road test after checking and clearing the codes and memory, set the scanner up to monitor the fuel trims, front O2 and calculated load. Before you hit the road, make sure the O2 and short-term trim is switching as expected. On the road, do a couple of wide-open accelerations, making note of the calculated load; it should a 90-plus percentage. If not, take a close look at the air sensor for contamination.

The sensors can be carefully cleaned — and we’re having good luck with some of the products designed for the job — but be gentle as they are fragile. We look at cleaning the sensor as part of the diagnostic process and recommend a new sensor to ensure a successful repair, so be sure to note that on the invoice to prevent any misunderstanding.

Once the problem is diagnosed and repaired, make sure that the bolts holding the sensor to the air filter housing are tight, that the air filter box is not allowing unfiltered air into the sensor, and that the air filter element is of high quality and in good condition. There have also been reports of the sensor housing bolts coming loose, letting dirt into the air stream, so be sure all the air is being filtered.

A P0300 series code indicating misfires has us moving to the ignition system. Many times, a misfire code will have a driveability complaint associated with it, but either way the diagnostic strategy is similar. If the miss is always evident, it shouldn’t take you long to figure it out. Be sure the engine is mechanically sound with good compression and spark plugs that are in good condition.

While you’re there, take a good look at the wires for any cracking or indication of carbon tracking; look closely for pinholes burnt through the plug boots that would let the spark get to ground. If any oil is evident, replace the valve cover gasket with spark plug tube seals. If the oil is deep enough, that could very well be the cause of your problem. Grab your stethoscope and be sure the injectors are opening and closing by listening for that distinctive click with the tool in the same location on each injector; they should all sound the same. I will say that we see very few problems with Mazda injectors.

Mazda has made the move to a distributorless ignition (DIS). Using waste spark, coil pack and coil-on-plug systems as the miles add up, none of the systems have been immune to problems. Plug wires have been the most common failures, but we’ve seen some coils fail as well. In the best case, the OBD II system is reporting the offending cylinder, sending you in the right direction. If not, it can be tough to pinpoint the offending cylinder, especially if it’s an intermittent problem.

While you can use a low-amp probe to look at the coils, it can be difficult to catch the problem. On a higher mileage, four-cylinder car that uses two coils and two wires, I would tend to replace all the components. On coil-pack cars, history tells us wires are the more common failure. With six-cylinder, coil-on-plug cars where the cylinder can’t be pinpointed, you have to decide if you should change them all or wait for the failure to become more evident. That decision would be between you and your customer. On the models that require manifold removal for access, we always recommend replacing all the coils and plugs while we’re there.

If you’re diagnosing a driveability complaint on older cars with a distributor, check the plug wires first. Even if there is no apparent misfire, an open plug wire will send electrical forces bouncing around under the hood, causing driveability problems you wouldn’t expect from a bad wire.

The other weak link in the ignition system is the distributor itself. You may have a complaint that the car cut out, but when it was started back up, it seemed fine. Or maybe it just cut out and wouldn’t start. There are no serviceable parts in the Mazda distributor, so if you have a no-spark condition that is traced to one of the components housed in the distributor, the unit will have to be replaced.

Actually, on some models, the igniter is available, but it costs as much as a rebuilt distributor. The aftermarket has high-quality rebuilt units available, but they don’t all come with the O-ring. You may be able to carefully replace the old one, but it’s much safer to have a new one on hand. When the time comes to put on the timing light, be sure to check your service information for any special procedure required to set the base timing.

One thing holds true no matter what ignition system is used: If you ignore a misfire code for any length of time, you will be faced with a P0420 catalytic converter efficiency code. Again, using the data side of the scanner, we want to take a look at the front and rear O2 sensors to see if the cat is doing its job. Unfortunately for the customer, we usually see the sensors mirroring each other, telling us the misfire has damaged the cat converter. If the front sensor were switching nicely while the rear stayed steady, we’d know the cat’s doing its job. Keep in mind that the cats are covered with an eight-year, 80,000-mile warranty.

There are a couple of things I’d like to share on the later model cars and trucks that will save you some headaches. There have been some reports of wiring harness failure in the popular Tribute model that could shut down an injector or coil on acceleration and set the appropriate misfire code. The failure is usually within a couple of inches of the ECU; a shake and wiggle test will confirm the problem. But with the wire breaking inside the insulation, it would be time well spent to check a pin chart to better identify the suspected wires.

The other is a problem with the 2.3 L engines used in the mid-2000 Mazda 6s. These timing chain-equipped engines have been having a problem with the cam timing shifting when the oil level gets extremely low. In a perfect world, everyone checks the oil and gets timely oil changes, but we all know that’s not always the case.

These engines use a taper to hold the cam gears to the cams while this system allows more precise timing. It’s apparent the cams are seizing momentarily due to the lack of lubrication and the tapers are slipping. Readily available are special tools required to re-time the camshaft, but keep in mind that these are interference engines that won’t tolerate much misalignment. Just remember that the cam timing can be an issue if faced with a no-start condition.

As I’ve said in all my articles lately, you can no longer rely just on what you know to effectively diagnose and repair the modern automobile. With the amount of inherent technology, from controlled-area networks, to reflash technology, to simply reprogramming keys, I’d be willing to say there’s no way for an independent shop to stay in the loop.

The good news is that there is a lot of good information available to help us make the right choices for our shop and customers. But  you have to have Internet access that will let you tap into this knowledge base. If you’re still trying to get by without it because you think you can’t afford it, believe me when I say it’s costing you more trying to do it by the seat of your pants



Students in the Cleveland, OH-based Ohio Technical College (OTC) High Performance and Racing program teamed with Mazda last August to build a race car and serve as its pit crew for The Longest Day of Nelson: 24-Hour Endurance Event.

The vehicle was prepped for Nelson, the world’s oldest 24-hour endurance race for amateur racers of production-based SCCA showroom stock automobiles that premiered in 1980, and students who built the car from the ground up had a fantastic opportunity to experience the entirety of careers available within the high performance and racing industries.

A total of 70 students from three different classes have worked on the MX-5, which placed 2nd in their class and 3rd overall in the Nelson’s Ledges event. The youthful pit crew worked on everything from installing roll cages to maintenance on the track.

“Our students were taught professionalism, sportsmanship and how to work with a team, which is the educational experience of a lifetime,” says Tom King, director of enrollment management at OTC. “They represented Ohio Technical College to the highest level, and for many this event was just the beginning of their racing careers.”

Mazda was the first import carmaker to get involved with OTC, providing the school with a complete MX-5 body in white, along with MAZDASPEED components, to allow students to build a race car in the same manner as professional race shops. Students began the project with a 2007 stock MX-5, which was upgraded with the following aftermarket performance components: Koni coil-over suspension; Wiseco pistons; ACT racing clutch; Enkei RPF1 wheels; Carbotech brake pads; Sparco Pro 2000 race seat and 6-point safety harness; Staubli dry-break refueling system; and MAZDASPEED competition radiator and headers.

The High Performance and Racing Program — currently a 12-week course — now has 60 students enrolled. But big changes are ahead for the program, which is being enhanced this summer into a full, 18-month program solely focused on high performance, motorsports and race technology.

“Within the 12-week program, you can only get an overview of all the intricate automotive components and technologies within this segment of the industry, so we’re excited to be able to teach a curriculum that develops more technical students who are capable of going out and getting jobs in racing venues,” notes Ed Hofstrom, high performance director at OTC, who adds that former students are working with Skip Barber Racing and machine shops across the country, for example.

OTC’s High Performance and Racing course currently provides intense training on the following subjects: High Performance Fuel Systems; High Performance Ignition & Braking; Race Car Suspension Development; Forced Air Induction; HP Cooling, Lubrication and Fuels; and HP Automatic Transmissions.

To learn more about the High Performance and Racing Program or other areas of OTC, visit and

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