Hi
We have a 2016 (new to us though) Mazda 2 from a mazda main dealer; so comes with warranty.
It is being driven by my learner son predominantly. A couple of times the check engine light has come on and using "car scanner" obd scanner I can see this is the P0302 (Cylinder 2 misfire) code.
The first time it happened he was stalling a lot, so I assume it was related and just cleared the code. It has happened again though - and although he did stall the car once it was just the once and his driving / starting / stopping is much smoother.
Could this be related to stalling the vehicle or should we get it checked in for a once over at the garage?

 

The DTC is the first step to diagnosis a problem. When you so called and miss understood cleared the DTC all you did was turn of the dash check engine light. This has always been a bad misconception on forums and internet info. The DTC will remain in the ECU permanent file until the cause is correct and if not the check engine light will return after 1-5 engine cycles. You can also check the IM monitors which one or more will be in fault as well until the original cause was corrected. So no passing an emissions test.

Try to resist randomly replacing parts unless you confirm they are failing or failed. Guessing and changing unconfirmed failed parts could and often does create additional problems added to the original unresolved problems.

 

good advice but my original question is whether repeated stalls (poor clutch control) could lead to this error - and so nothing to worry about - …or whether it needs diagnosis at the garage?

 

Sorry but you are asking a question that has nothing form me to even accurately say yes or no or maybe etc.
In terms of generality with any vehicle type manufactured all of the above with sensors reporting to the ECU can and should cause a check engine light and multiple engine codes as well as a in fault I/M section of monitors.

As I said someone that has a quality Diagnostic Scanning tool (NO BLUE TOOTH) and knows how to use it (not a average DIY) needs to go in and look at what is being reported and in files in the ECU.
Any forum or internet advise had better be followed with your adviser signing of with credentials like ASE (or similar accredited certification)! Otherwise, you are likely not going to be getting the best advice.


ASE

 

2016 Mazda 2 are common for excessive carbon build up on the valves.

Apart from swapping the spark plug & coil to other cylinders a compression test and cylinder leak down test can point to carbon build up ( otherwise you would need to remove the intake manifold )

 

Hello, Jabes.

An ICE (internal combustion engine) requires three things to operate properly:
- Air
- Fuel
- A combustion catalyst (EG: A spark or increased pressure until detonation occurs)

The P030X (1 = cylinder 1, 2 = cylinder 2, ...) is only notifying you of just that: A misfire has occurred.

The A/F ratio might be to blame. This could be caused by (any combination of): The mass air flow sensor (MAF), the fuel pump, the fuel pressure regulator, a fuel injector, an oxygen sensor (O2 sensor) or the powertrain control module (PCM or 'main central computer of the entire system of systems'); this is not an all-inclusive list and only pertains to the AFR (air-fuel ratio).

The issue could be a mechanical issue (such as a vacuum loss or immobilized component or hindered component). If the cylinder head was warped at some point or the master cylinder head gasket was damaged around that cylinder: That may cause oil and/or antifreeze to mix in the cylinder and affect the AFR or the spark itself thus producing a misfire.

The cylinder's related cylinder head's set of valves may also be to blame; if they are not seating (creating an airtight vacuum seal) properly: That cylinder might experience a vacuum loss (intake, exhaust or both) which could cause the cylinder to misfire (the knockback sensor that flags the cylinder; it can be found in front of the plastic manifold; it is a large, black rectangle plate with a hexagonical black sensor and single black wire; you can find it just by opening the hood and looking just beyond the hood latch and to the left).

The cause could be all of the above or it could potentially be an obstruction of air flow within any part between the air intake (the thing rectangular plastic intake located on the driver side that leads to the top of the engine where the air filter is located) and the cylinder itself; this path would include:
- The air intake tube
- The air filter
- The exhaust manifold
- The throttle body
- The EGR system (top center of the engine, on the back, by the firewall; cylinder and has an antifreeze radiator hose line running left-and-right beside it; the plunger can become oxidized and rust and cease to function)
- The valves (intake valves, namely; 4 total per cylinder; x2 intake and x2 exhaust)
- The cylinder bore itself (the engine block might be warped, the cylinder head might be warped, the cylinder head/crown might be damaged or potentially a cylinder ring)

There are other possibilities and although it might be a lot to digest: You can begin to eliminate each possibility through simple testing procedures.

If you do not own the tools: A friend, coworker or local auto parts store might lend them to you for free.

To test sensors of any kind (up to and including: Fuses and relays):
- A multimeter, alligator clips and electrical backprobes (for following wires from the electrical wiring harness to the points of interest such as the sensor)
- A compression tester (the cylinder head per cylinder and the cylinder head's valves)
- A blotter test (potential presence of oil in the antifreeze; there should be 100% none)

If you are able to obtain and OBD II reader (typically between $30 USD and $150 USD): You can monitor 'live data' directly from the sensors in real time which would be of great value.

To help rule out the fuel system: Read data regarding the rich and lean states (short and long term fuel trims).
To help rule out a vacuum leak: Read data regarding the absolute exhaust manifold pressure.
To help rule out a timing issue: Read data regarding engine timing retardation (a damaged, misaligned or worn component could be at fault).
To help rule out general ECU (engine control unit; sensors) issues: Read data on just about anything; the O2 sensors (bank 1, bank 2 come to mind).

The serpentine belt could be slipping, the serpentine belt tensioner, idler pulley or any, really could have seizing or failing bearings and it happened to be cylinder #2 that was instructed to fire during the failure (pendantic; chaos theory). The internal DOHCs might be out of alignment slightly due to warpage, the engine chain could be damaged at some part or the DOHCs might be missing a tooth, the harmonic balancer itself could be missing a tooth (which prevents the hall effect sensor from functioning), the crankshaft position sensor could be failing or it's air gap could be off somehow (either the harmonic balancer or the engine block as a whole or the timing chain cover itself warped or the sensor wiggled around somehow; it has one tiny bolt holding it in place), the camshaft position sensor may be failing (found beside the oil cap on the passenger side of the engine's top or engine valve cover), there are some other (pedantic) potential issues causing the engine cylinder to misfire.

I hope this helps! It is rather easy, cheap and fast to rule out the bulk of these issues. Not all sensors can be reliably tested (even with an oscilliscope); the crankshaft position sensor (CKP) and camshaft position sensor (CMP), namely due to being hall effect sensors; the remainder are testable in minutes.

Lastly: The transmission control module (TCM) might be to blame (pedantic); if the PCM believes the vehicle shifted from any gear into 'neutral': It may have very well cut off the spark to cylinder #2.

On that notion: The ignition coils (COP; coil-on-plug) or spark plug itself may simply be to blame for the misfires! The air gap on the spark plug or carbon build up (SeaFoam inducted through a vacuum line such as on the passenger side just by the alternator works great and is safe); both components (ignition coil and spark plug) can be tested very quickly and accurately with a multimeter.

I would post dozens of URLs, but here is the master head of the website instead:
- https://www.ma2auto.com/

You may feel free to read the "2007-2014 Mazda 2" pages as they mostly apply (95%+); testing sensors and expected input/output values and item locations are roughly the same.

Good luck! Let us know if you need help with anything or if anything changes and especially if you solve the issue (as posting the answer might help someone else, years later).

Thank you!