In Mazda MX-5 Mk3 NC cars, uneven engine operation, jerking, and similar issues occur quite frequently. Sometimes they are accompanied by error codes P0300, P0301, P0302, P0303, P0304. The causes can range from minor to serious, but any repair should start with proper diagnostics. For this purpose, software like Forscanwhich works with multiple interfaces (ELM327 USB and Bluetooth versions, ELS27, Tactrix Openport, and others), is suitable.
After connecting to the car (ignition on, engine does not need to be running), go to the Vehicle Test tab
and then select OBDII Mode 6 Test Results.
Note: Depending on the target market of the vehicle, the amount of available data varies. The version shown here is for the EU market. US-market cars have more comprehensive diagnostics and more parameters, while JP-market cars have fewer (e.g., no misfire count!).
Pay attention to the following parameters: Misfire Cylinder 1 2 3 4 Data.
For each cylinder, there are two parameters:
Misfire count for last/current driving cycle (calculated) – the number of misfires recorded during the last or current drive.
EWMA misfire counts for last 10 drive cycles - wykładniczo wygładzona średnia krocząca (Exponentially Weighted Moving Average) of misfires for the last 10 drive cycles. This is useful for locating recurring problems.
Ideally, all 8 values should be 0, but usually they are not. Single misfires occur intermittently in the MX-5 Mk3 NC, and Misfire Count values in the range of 1–3 (or sometimes 5) are generally nothing to worry about. Unfortunately, the NC’s onboard diagnostics are far from perfect.
Repeated high values on specific cylinders indicate which cylinder is problematic. In general, misfires can be caused by: Ignition system issues (wiring, spark plugs), Injector problems, Intake system leaks, Valve leaks (sometimes adjustment is sufficient; occasionally valve damage/wear)
If the misfire count is noticeably above 0 and your NC is from 2006–2007, a common issue is the coil harness. The early version of the harness looks like in the photo below. Thick, stiff, heavy cables between the fuel rail and ignition coils often break near the coil due to vibration, temperature, and age. Initially, the engine may only occasionally run poorly, and this can happen randomly. Sometimes no OBD II error is triggered.
Solutions:
- Replace with revised engine harness
- Repair the harness, e.g., using my plug and wire kit
The improved harness, installed from around 2008 onward, has a lightweight, sleeved section between the fuel rail and the coils.
With this updated harness, problems with the wiring are usually resolved.