2013~2016: OBD2 scanning - when tracking down P0300 random misfire

[GFrosty]

What specific OBD2 values do you find most valuable, in the tracking down of a P0300 Random Misfire Detected condition?

Suggestions on which combinations of OBD2 scanner indicators to focus on?


Hopefully, some who have solid experience using the OBD2 to identify sensor outputs and conditions will offer some guidance.

Will be configuring an OBD2 scanner, soon, to attempt to identify the cause(s) of this occasional condition on my vehicle. (It's a 2016.5 CX5 GT AWD with almost 130Kmi on the clock, with fairly good maintenance.) Hasn't been specific to a cylinder, just the generic P0300:00 code. And it hasn't occurred recently, within the last few weeks.

Recent maintenance/changes (post-P0300 CEL displayed): new spark plugs; new ignition coils.

According to some troubleshooting guides I have seen (not Mazda vehicle specific), the possible causes of the P0300:00 condition could include:

• Spark plug malfunction -- have swapped-in new spark plugs
• Ignition coil malfunction -- have swapped-in new ignition coils
• Ignition coil related wiring harness or connector malfunction
• Ignition system -- improper operation
• Fuel injector malfunction --
• Fuel injector(s) -- improper operation --
• Fuel injector related wiring harness malfunction
• Erratic signal to PCM
• CKP sensor signal malfunction (crankshaft position sensor) --
• ECT sensor No.1 signal malfunction (engine coolant temp) --
• MAF sensor signal malfunction (mass air flow) --
• MAP sensor signal malfunction (manifold absolute pressure) --
• IAT sensor No.1 signal malfunction (intake air temp) --
• APP sensor signal malfunction (accelerator pedal position) --
• TP sensor signal malfunction (throttle position) --
• VSS signal malfunction (vehicle speed sensor) --
• Related connector or terminals malfunction
• Related wiring harness malfunction
• Poor drive belt assembly or adhesion of oil -- 35Kmi since last swap; haven't seen/felt anything on belts
• Drive belt auto tensioner malfunction -- has 35Kmi since last swap; haven't heard any strange noises out of the thing
• Air leakage from intake air system (between intake manifold and cylinder head) -- air intake from airbox through the throttle body seems to have good connections, hose condition, and no apparent leaks; throttle body itself has a new gasket
• Engine malfunction
• Insufficient engine compression -- haven't done recent compression and leak-down tests
• Engine coolant leakage to combustion chamber -- coolant level remains consistent
• PCM malfunction

Initially, it looks as though the following are a good place to start, in this UltraGauge MX scanner:

Page 1 settings:

1. RPM
2. MPH
3. ECT - engine coolant temp
4. MAF Sensor 1 g/s
5. MAF Sensor 2 g/s (calculated)
6. MAP (intake manifold absolute pressure) PSI
7. IAT (intake air temp) ºF
8. ECM battery voltage

Page 2 settings:

1. APP 1 (accelerator pedal position 1) %
2. APP 2 (accelerator pedal position 2) %
3. Throttle Position % CMD
4. Throttle Position 2 % absolute
5. Absolute Throttle Position 1 %
6. Relative Throttle Position %
7. Vehicle Speed MPH

Page 3 settings:

1. Can use additional outputs/conditions if they can assist in nailing-down "random misfire" conditions.

 

[AL Cx5]

The trouble shooting list by Mazda is pretty much like all cars that have a random misfire.

To fix this will require one do some wrenching vs. scanning. The list didn't mention scan for the solution.

How many miles on your spark plugs? That's #1 on the list, start with new spark plugs if yours are not fresh. IF the plugs have a lot of miles, I would suspect they are failing. This is a cheap, easy fix.
Have you swapped some coil packs around?
Check the wiring and connectors.

If you can scan air to fuel ratio, go for a drive and log AFR. You might see it fluctuate, like it has a bad injector. This is not likely on a Mazda unless a lot of trash got into the fuel tank...

 

[GFrosty]

To fix this will require one do some wrenching vs. scanning. The list didn't mention scan for the solution.

Understood. Though, scanning quite likely can help identify whether some sensed condition is wonky, suggestive that a part might have an issue or be going bad.

How many miles on your spark plugs? That's #1 on the list, start with new spark plugs if yours are not fresh.
Have you swapped some coil packs around?
Check the wiring and connectors.

As indicated, the plugs and ignition coils are new. Smoother, but the CEL and same P0300 has occurred since the swap (but subsequently cleared up).

If you can scan air to fuel ratio, go for a drive and log AFR. You might see it fluctuate, like it has a bad injector. This is not likely on a Mazda unless a lot of trash got into the fuel tank...

Yes, the scanner supports air/fuel ratio scanning. Via a "AFR Lambda commanded ratio" monitor and a "Wideband O2 sensor lambda" monitor. Reading up on this scanner's config of those gauges and how to use them.

 

[Jmaz]

That's a massive list of potential offenders. Where do you even start. I didn't see you mention experiencing any no start/hard start situations but if you did, a failing crankshaft position sensor can cause those as well as misfires and poor running/lack of power on an intermittent basis.

 

[GFrosty]

I didn't see you mention experiencing any no start/hard start situations but if you did, a failing crankshaft position sensor can cause those as well as misfires and poor running/lack of power on an intermittent basis.

While the stumbling and CEL had been occurring, on two occasions there were instances of a harder start. Starts that took another second longer than normal, followed by immediate hard stumbling for another second, then normal idle and occasional 'stumbling' condition.

Fortunately, the crankshaft position censor (CKP) is easily accessed. Per the workshop manual, metal shavings on the CKP could easily cause intermittent signalling that could impact idling, engine control, ignition timing, fuel injection. Can ensure it's clean and undamaged, easily replacing if needed.