P0300 is a very common OBDII codes that occurs with the Toyota Mirai. It stands for:
P0300: Random, Multiple Misfire Detected
P0300 is certainly a cause for concern, and can be a threat to the drivability of the Mirai.
It will often be accompanied by codes P0301-P0308. These particular codes also indicate that there is a misfire detected, but they specify the specific cylinder. If you have P0300 and one of these other codes, the cylinder indicated by the cylinder specific codes would be where to start looking for your problem.
Toyota Mirai P0300 Definition
P0300 is a general and random misfire code, which means that the code is not going to tell you which cylinder is causing the misfire. Something is causing cylinders to misfire at random, which is a lot less cut and dry than a cylinder specific misfire code.
Mirai P0300 OBDII Code Symptoms
- The P0300 Toyota trouble code will trigger the Mirai’s service engine soon light.
- The vehicle itself may begin to run rough. It depends on how serious the misfire is.
- The vehicle may produce excess vibration, especially at lower RPM
P0300 Trouble Code Diagnosis- Toyota Mirai
There are quite a few things that can cause the P0300 trouble code to trigger the Toyota Mirai . Here are the most common problems that will throw the code. They are presented somewhat in order from most to least likely to be causing the code:
- Low Fuel Pressure– If there isn’t enough fuel getting to the engine, this will cause combustion to be less than optimal. Diagnosing low fuel pressure can be tricky. Typically, if you do have low fuel pressure, the vehicle will act fine when it doesn’t need a lot of fuel. But, it’ll sputter and act like it’s going to die at speed or under heavy acceleration. Here’s some information on how to tell if you have a bad fuel filter.
- Vacuum leak– If your Mirai has a vacuum leak, it can be very difficult for it to get the right air/fuel mixture. This will cause the cylinders to misfire and it’ll throw the P0300. Also, since a vacuum leak almost always affects each cylinder the same, you’ll typically get P0300 with it and not any cylinder specific misfire codes. Here’s a great article from Popular Mechanics on how to detect a vacuum leak. It’s easy (and kind of fun) to chase one down. Popular Mechanics: How to find a vacuum leak.
- EGR Problems– If the EGR system is not able to recycle the engine gasses right, it’ll throw P0300.
- Ignition Problems– Bad plug wires (if equipped), bad coil packs, and spark plugs can cause misfires to occur. This isn’t higher on the list because typically you’ll get a misfire in one cylinder specifically, and not a P0300 only. If you got a P302 or something similar with the P0300, it may be a good idea to check and see if there is any damage or failure from your ignition components. Here’s how to test a coil pack, how to tell if a spark plug is bad (video), and how to test plug wires (video).
- Cam or Crank Sensors– This one is very unlikely, but it does happen. If the ECU is not getting the right signal from these sensors, the vehicles timing is not going to sync up and it’ll misfire.
- Low Compression– If you have a leaking head gasket, bent valve, cracked head, etc.. that would cause compression to not be as high as it should, you’re going to get P0300. You should also feel the vehicle is down on power as well.
Most Common P0300 Fixes
A lot of the time, P0300 is going to be fixed by something obvious, such as an EGR leak. When it’s not glaringly obvious what is wrong, a tune up is a great place to start.
Replacing the spark plugs, coil packs, and plug wires is not typically difficult or costly.
Is P0300 a Serious Concern?
P0300 is cause for concern and left unfixed can get worse. Fixing the problem causing the code to fire can save you time and money and keep your Toyota from breaking down.
Toyota Mirai P0300 Diagnosis
P0300 is serious enough that you should take your vehicle in to have it looked at if you don’t know what you’re doing under the hood, but if you do, here are the recommended tools for getting the job done.
Fuel Pressure Tester: Actron CP7838 Professional Fuel Pressure Tester
Leak Down Tester: OTC 5609 Cylinder Leakage Tester Kit , black
Combustion Leak Tester: Block Tester BT-500 Combustion Leak Test Kit - Made in USA
Good luck diagnosing P0300 in your Toyota Mirai!
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