P0301 is a fairly common trouble code with the Toyota Mirai. It’s an OBD2 code and indicates that your car’s first cylinder is misfiring.
P0301 is certainly a cause for concern and should be considered a threat to the drivability of your Mirai.
Fixing P0301 should be considered a high priority. Unlike a lot of the OBDII codes, this code has to do directly with engine combustion. It also can cost money to ignore it, since driving with a misfire can damage the Mirai’s catalytic converter.
The most common fix for P0301 is to replace the spark plugs, coil packs, or (if equipped) plug wires.
Table of Contents:
P0301 Meaning: Cylinder 1- Misfire Detected
P0301 is a cylinder-specific misfire code, meaning it’s only the first cylinder that has a misfire. Cylinder one is typically the closest to the front of the engine.
On “V” configuration engines, it’ll usually be the first cylinder on the head closest to the front of the engine. A quick google image search will confirm the correct cylinder (include your Mirai’s model year).
In order for an engine to burn fuel efficiently, each cylinder needs:
- Fuel– In the right quantity (around 14.7 parts air to one part fuel).
- Air– It needs to be metered by the mass air flow sensor or MAP sensor, so the powertrain control module (PCM) knows exactly how much air to let into the combustion chamber.
- Spark– The spark plug needs to fire at the right time and at the right temperature.
- Compression– The air-fuel mixture must be compressed during the engine’s power stroke.
If one of the above variables is off, your Mirai’s first cylinder will misfire, or it won’t fire at all.
Mirai P0301 OBDII Code Symptoms
Here are the most common symptoms of P0301 in the Toyota Mirai.
- The P0301 trouble code will trigger your Mirai’s check engine light.
- The car itself may begin to run rough. It depends on how serious the misfire is.
- The vehicle may produce excess vibration, especially at lower RPM.
- Fuel mileage may suffer.
- You may smell raw gas coming from the tailpipe.
- The Mirai may backfire.
P0301 Trouble Code Causes: Toyota Mirai
There are quite a few things that can cause the P0301 trouble code to trigger the Toyota Mirai.
Here are the most common problems that may cause it. They are presented somewhat in order from most to least likely to be causing the code:
- Bad Spark Plugs– A fouled spark plug is the most common cause of P0301. Look at the electrodes and see if they are in good shape. Most vehicles now come with iridium plugs that need to be changed very infrequently. That being said, a bad spark plug is the number one offender when it comes to P0301. Here’s a great video on verifying a spark plug is bad.
- Spark Plug Wires– On most modern engines, the plug wires are not nearly as long as they once were (if your car even has them), but they can still go bad. Here’s how to tell if your plug wires are bad (video).
- Coil Packs– Coil packs rarely go bad, but when they do, they can cause P0301 in your Toyota Mirai. Replacing a set can be very expensive. Here’s how to test them.
- Bad Fuel Injector– If you have a fuel injector that has failed, it won’t be able to properly atomize fuel, and you’ll get P0301. Here’s a good video on how to diagnose an injector; it can be a little tricky. You’ll likely see P0171 with a bad injector.
- Vacuum leak– If your Mirai has a vacuum leak, it can be difficult for the PCM to get the right air/fuel mixture. This will cause the cylinders to misfire, and it’ll throw P0301 if the leak is around that specific cylinder on the intake manifold. Popular Mechanics: How to find a vacuum leak. You’ll likely see P0171 and P0300 when there’s a vacuum leak.
- Cam or Crank Sensors– This one is unlikely, but it does happen. If the ECU is not getting the right signal from these sensors, the vehicle’s timing will not sync up, and it’ll misfire. You’ll likely get P0300 and a cam/crank correlation code too.
- Mechanical Issue– If your Mirai has a leaking head gasket, bent valve, cracked head, etc… that would cause compression to not be as high as it should be, you’re going to get P0301. You should also feel that your car is down on power as well.
How to Fix P0301 in the Toyota Mirai
If P0301 is the only code you get when you plug an OBD II scanner into your Mirai, this section should help you determine what is causing the code.
If you have multiple codes with P0301, jump down a section.
1. Swap Test
There’s a quick and easy test to determine what is causing your Mirai’s cylinder one to misfire. Swap ignition parts to another cylinder. Here’s how to do it.
- Identify the first cylinder.
- Remove the spark plug.
- Swap it with the most convenient cylinder to access’s spark plug.
- Clear your Mirai’s trouble codes with a scanner.
- Start the engine and wait for the check engine light to come back on. If it changed to a different P030X code, the plug was bad.
- If nothing changed, do this for the coil packs and plug wires (if equipped).
- If the code stays P0301, you’ve ruled out ignition-related issues.
Here’s what to look for when the swap test didn’t change your car’s misfire code number.
- Check the wiring harness going to the cylinder one coil pack. If it’s damaged or loose, repair it. Ignition wiring is a common rodent damage area.
- Check for a vacuum leak.
- Verify injector one is working (there’s a link to how to test one above).
- Do a compression test.
- Perform a leak-down test.
P0301 + Other Codes
P0301 + P0300
The most likely reason your Mirai will get P0300 and P0301 simultaneously is faulty spark plugs.
P0300 indicates random multiple misfiring, which means that multiple cylinders are misfiring at the same time.
If your Toyota Mirai has P0300 and P0301 error codes, it’s best to diagnose why cylinder one is misfiring and see if that will fix the P0301 code.
Try doing the swap test from the previous section. Make note of the condition of BOTH spark plugs as you swap them. If they look fouled, new plugs will likely clear this code.
P0301 + P0171
Look for a vacuum leak around the first cylinder or a bad fuel injector.
P0171 is one of the most common trouble codes there is. While there are many potential causes for P0171, the most prevalent is a vacuum leak.
When your Mirai has P0301 and P0171 together, it’s often caused by a clogged or underperforming fuel injector.
P0301 + Other Cylinder Misfire Codes
Treat P0301 with these codes like it has P0300.
Treat P0301 with these codes like it has P0300. Suspect an ignition-related cause or a vacuum leak.
Is P0301 a serious concern?
P0301 is cause for concern and, left unfixed, can leave you stranded. Your Mirai will be virtually undrivable if the first cylinder stops firing altogether. The raw fuel can also damage the catalytic converter.
Can you drive your Mirai with P0301?
We do not advise driving your Mirai with P0301. It can cause damage to your car.
Is P0301 hard to repair?
Most of the time, P0301 is not hard to repair, as you’ll be swapping out ignition parts. It’s a great first-time project for a shade tree mechanic. If the problem ends up not being ignition related, that’s when you might have to bring it into a shop.
Replacing the spark plugs is the most common fix for P0301 in the Toyota Mirai. There are other causes, but the swap test can help narrow them down considerably.