If your Honda Civic has the P0304 trouble code, you should know that it’s about as common of a code as there is. It’s a generic OBD IIcode (same meaning for all vehicles) that indicates that your car’s fourth cylinder is misfiring.
P0304 is a drivability concern and should be dealt with right away. If it’s bad enough to cause your Civic’s check engine light to flash, avoid driving your car altogether until you find the misfire and repair your engine.
Repairing P0304 should be considered a high priority. Unlike a lot of the OBDII codes, this code has to do directly with engine combustion, and driving with a misfiring engine can damage your Civic’s catalytic converter.
The most common repair for P0304 is changing the spark plug in cylinder four.
Table of Contents:
P0304 Definition: 4’th Cylinder- Misfire Detected
P0304 is a cylinder-specific misfire code, meaning only your Civic’s fourth cylinder has a misfire.
Before you can even diagnose P0304 in your car, you need to find the fourth cylinder’s location.
On “V” configuration engines, it’ll usually be the second cylinder on the head further away from the front of the engine. On inline engines, the fourth cylinder will usually be the fourth in from the engine accessories.
A quick Google image search will confirm the correct cylinder (include your Civic’s model year and engine type), or they can print it up for you at most parts counters.
In order for an engine to burn fuel efficiently, each cylinder needs the following:
- Fuel– In the right quantity (around 14.7 parts air to one part fuel).
- Air– It needs to be metered by your Civic’s 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 (diesel excluded).
- Compression– The air-fuel mixture must be compressed during the engine’s power stroke.
If one of the above variables is off, your Civic’s fourth cylinder will misfire, or it won’t fire at all.
Honda Civic P0304 Symptoms
Here are the most common symptoms of P0304 in the Honda Civic. There may be no noticeable symptoms at all.
- Check engine light
- Your car may run rough, be down on power, and stall.
- Your Civic may produce excess vibration, especially at lower RPM.
- Fuel mileage may suffer.
- You may smell raw gas coming from the tailpipe.
- The engine may backfire.
P0304 OBD II Code Causes: Honda Civic
Here are the most common P0304 Honda Civic causes; they are presented somewhat in order from most to least likely to be causing the code in your car:
- Bad Spark Plugs– A fouled spark plug is the most common cause of P0304. Look at the electrodes and see if they are in good shape. Your Civic likely has 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 P0304. 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 P0304 in your Honda Civic. 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 the fuel, and you’ll get P0304. Here’s a good video on how to diagnose an injector; it can be a little tricky. You’ll likely see P0171 and/or P0204 with a bad injector.
- Vacuum leak– If your Civic 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 P0304 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 Civic 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 P0304. You should also feel that your car is down on power as well.
P0304 Fix: Honda Civic
If P0304 is the only code you get when you plug an OBD II scanner into your Civic, this section should help you determine what is causing the code.
If you have multiple codes with P0304, jump down a section.
1. Swap Test
Here’s a quick and easy test to determine what is causing your Civic’s fourth cylinder to misfire. Swap ignition parts to another cylinder. Here’s how to do it.
- Identify cylinder four.
- Remove the spark plug.
- Swap it with the most convenient cylinder to access’s spark plug.
- Clear your Civic’s DTC 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 P0304, you’ve ruled out the most common causes of this code.
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 four coil pack. If it’s damaged or loose, repair it. Ignition wiring is a common rodent damage area.
- Check for a vacuum leak.
- Verify the fourth injector is working (there’s a link to how to test one above).
- Do a compression test.
- Perform a leak-down test.
P0304 + Other Codes
P0304 + P0300
The most likely reason your Civic will get P0300 and P0304 simultaneously is faulty spark plugs.
P0300 in the Honda Civic indicates random multiple misfiring, which means that multiple cylinders are misfiring at the same time.
If your Honda Civic has P0300 and P0304 error codes, it’s best to diagnose why the fourth cylinder is misfiring and see if that will fix the P0304 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.
P0304 + P0171
Look for a vacuum leak around the fourth cylinder or a bad fuel injector.
While there are many potential causes for P0171, the most prevalent is a vacuum leak.
When your Civic has P0304 and P0171 together, it’s often caused by a clogged or underperforming fuel injector. Look for a vacuum leak at the intake around cylinder four.
P0304 + Other Cylinder Misfire Codes
Treat P0304 with these codes like it has P0300.
P0304 will often be accompanied by codes P0301, P0302, P0303, P0305, etc… These codes indicate a misfire in cylinders one, three, four, and five, respectively.
Treat P0304 with these codes like it has P0300. Suspect an ignition-related cause or a vacuum leak.
Is P0304 a serious concern?
P0304 is cause for concern and, left unfixed, can leave you stranded. Your Civic will be virtually undrivable if the fourth cylinder stops firing altogether. The raw fuel can also damage the catalytic converter.
Can you drive your Civic with P0304?
We do not advise driving your Civic with P0304. It can cause damage to your car, particularly if the check engine light is flashing.
Is P0304 hard to repair?
Most of the time, P0304 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 plug or coil packs is the most common fix for P0304 in the Honda Civic. There are other causes, but the swap test can help narrow them down considerably.