You need to solve your misfire code as soon as possible to avoid damaging engine internals and expensive catalytic converter because unburned fuel is going through your whole engine and exhaust. This will also prevent you from passing state emission inspection.
I studied misfire problems on more than 120 different vehicles and found out the most common causes and now I prepared you diagnosing guide to follow based on how real mechanics fixed their misfire issue.
P0300 quick overview
| Meaning | P0300: Random/Multiple Cylinder Misfire Detected |
| Is it serious? | Yes, this code is serious as it can lead to engine damage and expensive catalytic converter failure if not addressed promptly. |
| Possible causes | -Faulty spark plugs or ignition coils -Fuel system issues (injectors, fuel pump, or fuel pressure regulator) -Engine mechanical problems (valves, pistons) -Timing issues (timing belt/chain off) -Vacuum leaks -Faulty sensors (MAF, O2) |
| How to diagnose? | Read OBD2 Codes: Check for additional codes that may indicate specific causes. Inspect Spark Plugs/Coils: Swap spark plugs and ignition coils to see if the misfire moves to another cylinder. Check Fuel System: Test fuel pressure and inspect injectors. Perform Compression Test: Check engine compression to rule out mechanical issues. Inspect Timing: Verify engine timing is correct. Check for Vacuum Leaks: Inspect intake manifold, hoses, and gaskets for leaks. |
P0300 Meaning
The P0300 code means your engine isn’t firing correctly in more than one cylinder, which can make the engine run rough. It’s important to fix this soon to avoid serious damage to your engine and exhaust system.
What causes the P0300?
I dug up 120+ old service posts directly from a mechanics who experienced the P0300 issue and reported the fix afterward.
- Spark (29%)
- Fuel delivery (24%)
- Engine damage (23%)
- Timing (17%)
- Air intake (14%)
- Others (17%)
There are a lot of causes for misfires so don’t rush with changing spark plugs and ignition coils, before you diagnose the issue.
FOLLOW THIS to diagnose P0300!
1. Read other OBD2 codes (real cases)
If you are getting other OBD codes, you have another clue pointing you at the faulty part causing this misfire issue. I analyzed 100+ misfire cases based on the reported fix directly from a mechanic who worked on the vehicle. These were the most common code combinations if there were any present other than P0300.
P0300 + no start condition
Car #1: 1996 Volkswagen Jetta GL 2.0L
Problem: No start, P0300, P0304
Diagnosing: After checking the basic causes of no start like no fuel delivery or lack of spark, the technician used the oscilloscope and tested the ignition switch which was fine. Then he moved to ECM tests. If the coil output stage lacks a square wave, the crank sensor tests fine, and all power is present as expected, it suggests a faulty ECM as the underlying issue.
Fix: Replace ECM (be careful as this technician replaced for used ECM and the car still didn’t work until trying a different one). Used installed parts can also be faulty.
Car #2: 2004 Pontiac Grand Am GT 3.4L
Problem: No start, P0300, P0645
Diagnosing: The person is seeking help with a problematic car that has intermittent starting issues. They have ruled out ambient temperature and sun exposure as factors, and there are no related error codes. Only other code is for A/C circuit.
Fix: Replace the A/C fuse fixed the intermittent starting issues and P0300. If you are stuck, solve other issues first if they are simple as changing the fuse, you never know like in this case.
P0300 + Lean bank codes (P0171, p0174)
Car #1: 1999 Chevrolet Silverado 1500 5.3L
Problem: Check engine, P0300, P0171, P0174
Diagnosing: The technician tried to replace a bunch of parts like the MAF sensor, oxygen sensors, and fuel pump but without success. The cracked air intake box was discovered later which can be causing the lean condition as there is more air coming into the engine, but the fuel amount stays the same.
Fix: Fixed by repairing vacuum leaks. Check for this if you have a similar problem.
Car #2: 2002 Toyota Camry SE 2.4L
Problem: Check engine, P0300, P0171, P0302, P0303
Diagnosing: This time the technician went straight to air intake since this code combination + high fuel trims numbers is most likely caused by a vacuum leak. The leak wasn’t visible at first but the technician replaced intake gaskets.
Fix: Fixed by replacing gaskets on the air intake.
Car #3: 2002 Chevrolet Blazer LS 4.3L
Problem: Poor idle, misfire P0300, P0171, P0174
Diagnosing: The technician checked the fuel trims which were looking normal so there probably aren’t any vacuum leaks. A lean condition is when there is less fuel and more air coming into the engine so if the problem is not the air intake system, the technician started to look into the fuel system.
Fix: Fixed by cleaning fuel injectors.
P0300 + Lean bank: There is more air for the same amount of fuel. You most likely have a vacuum leak, or there is a problem in your fuel system.
P0300 + Rich condition (P0172, P0175)
Car #1: 2003 Toyota Avalon XL 3.0L
Problem: Poor idle, misfire P0300, P0172, P0174, P0325
Diagnosing: The technician replaced a bunch of parts from the oxygen sensor to the fuel pump. The problem was however in the engine’s timing. It was 1 tooth off.
Fix: Fixed by repairing the timing kit.
Car #2: 1996 Chevrolet C2500 Cheyenne 5.7L
Problem: Misfire, P0300, P0172, P0175
Diagnosing: The technician checked for misfires with the OBD2 scanner and discovered, it is present only at idle. When he increased the RPM, the misfire stopped immediately.
Fix: Fixed by changing fuel pressure regulator
Car #3: 2015 Chevrolet Silverado 1500 LT 5.3L
Problem: Misfire, P0300, P0172, P124c (injector control circuit short)
Diagnosing: The technician suspected an electrical issue in the vehicle’s injector system but couldn’t replicate the problem during testing. The issue seemed to be related to a leaking injector causing over-fueling.
Fix: Fixed by changing fuel injectors.
P0300 + Rich bank: There is more fuel for same amount of air. It can be caused by misfire. The most common cause was in the fuel system according to my research.
P0300 + EGR codes (P0400, P04XX)
Car #1: 1998 Chevrolet Malibu LS 3.1L
Problem: p0300- random misfire
p0404- EGR feedback signal out of range
p1404- EGR 2 position error – EGR closed performance
Diagnosing: The car is misfiring mostly at idle and the technician thinks the problem is in the fuel system so he cleans the fuel injectors. Didnt help, the fault was exactly where the fault codes are pointing. EGR.
Fix: Replacing the EGR valve did the fix.
Car #2: 2000 Toyota Camry CE 2.2L
Problem: Check engine, P0300, P0401
Diagnosing: The technician encountered an insufficient EGR (Exhaust Gas Recirculation) error code, P0401, in a Toyota. How Toyota set this code by cutting EGR at a specified RPM and observing the MAP (Manifold Absolute Pressure) change.
Fix: The problem turned out to be a faulty VSV (Vacuum Switching Valve) for the EGR, located under the intake on the back of the block.
Car #3: 2001 Chevrolet Silverado 1500 4.8L
Problem: Poor idle, misfire P0300, P0404, P0405
Diagnosing: The technician tried to block the EGR, but the misfire didn’t change or disappear at all. Now he knows, the problem must be somewhere else and since this model have often issues because of intake gaskets, he replaced them. One bank is working fine now, but the other bank still has one cylinder misfiring.
Fix: Fixed by cleaning fuel rail and injectors.
P0300 + EGR code: If you are unsure about what is causing the problem, try blocking the EGR. If the misfire disapears, the fault must be in EGR system.
p0300 + CAT codes (P0420, P0430)
Car #1: 2000 Chevrolet Suburban 1500 LT 5.3L
Problem: P0300, P0420
Diagnosing: The technician replaced the oxygen sensors and catalytic converter which solved a problem, but the customer returned with the same codes. It is more likely that the misfire is damaging the catalytic converter, eventually causing the P0420/P0430. That’s why it is important to fix misfires first.
Fix: The technician reported replacing the catalytic converter again for the OEM part as a fix, however, there is a high possibility that there were misfires damaging converters and the fault will be back soon.
Car #2: 2001 Mazda Protege ES 2.0L
Problem: Check engine, P0300, P0420
Diagnosing: This technician took the right approach and diagnosed the misfire before worrying about the catalytic converter. He was able to find a bad ignition coil by a simple swap test (swap coils if the misfire moves to another cylinder) and changed the coil.
Fix: The P0420 returned, indicating the catalytic converter is also damaged which was replaced as well.
P0300 + CAT code: Don’t replace catalytic converter unless you fixed the misfire first. Unsolved misfire will damage your new cat as well and they cost $500 – 2000.
p0300 + CAM/Crank sensors code
Car #1: 2001 Audi A6 Quattro 2.8L
Problem: P1347- bank 2 camshaft position sensor
Diagnosing: The technician removed the left cam cover, which revealed the absence of the chain tensioner upper rail cover. They noticed that the intake cam was misaligned, while the exhaust cam was fine. Though they were hoping to avoid removing the timing belt, it was necessary to replace both the timing belt and the intake cam.
Fix: Replace the timing kit.
Car #2: 2005 Audi A6 Quattro 3.2L
Problem: Check engine, P0300, P0346, P0391
Diagnosing: Same as the previous Audi, the issue was found in the timing chains as well which is causing a lot of misfire problems according to the study I did.
Fix: Replace the timing kit.
P0300 + CAM/CRANK codes: This is most likely caused by timing issue. Check your timing precisely. The chain/belt doesn’t have to be worn out, but it can be few teeth off and needs to be adjusted.
P0300 caused by bad ECM
Car: 2008 Pontiac Solstice 2.4L with P0300
Problem: Hesitation, Cuts out, Stall
OBD2 scan:
p0507- idle speed high
p0101- maf performance
p0106- map performance
p0116- ect performance
p0300- misfire
p0068- throttle body air flow performance
Diagnosing: Only clue was that the problem was temperature related since the vehicle switched from poor running to good condition once the engine got warmed up. The technician tried to replace the ECT sensor which didn’t work.
Fix: New ECM fixed the issue. Faulty ECM was verified on a cold engine by heating it up with a heat gun which instantly made the engine run OK.
2. Solve your other misfire code first
Maybe you aren’t getting code for another engine system, but you have code P0301 – P3012. They are pointing at the misfire at cylinder #1 to cylinder #12, based on your engine design.
If you have one of the following codes, fixing them will most likely fix your P0300.
Other misfire codes
If you have any of these codes present, click on them to get an idea of how to solve them quickly along with some more examples of real mechanics fixing this particular code.
| P0301 | Cylinder 1 Misfire Detected |
| P0302 | Cylinder 2 Misfire Detected |
| P0303 | Cylinder 3 Misfire Detected |
| P0304 | Cylinder 4 Misfire Detected |
| P0305 | Cylinder 5 Misfire Detected |
| P0306 | Cylinder 6 Misfire Detected |
| P0307 | Cylinder 7 Misfire Detected |
| P0308 | Cylinder 8 Misfire Detected |
| P0309 | Cylinder 9 Misfire Detected |
| P0310 | Cylinder 10 Misfire Detected |
| P0311 | Cylinder 11 Misfire Detected |
| P0312 | Cylinder 12 Misfire Detected |
| P0313 | Misfire Detected With Low Fuel |
| P0314 | Single Cylinder Misfire (Cylinder not Specified) |
| P0316 | Engine Misfire Detected on Startup (First 1000 Revolutions) |
A simple trick to solve the misfire on the specific cylinder is to simply swap spark plugs, ignition coils, and fuel injectors to another cylinder and try running the vehicle. If the misfire moves to another cylinder, you now know that one of these components is faulty and you solved the problem.
No other misfire codes
If you have only P0300 present, the fault will be in some system that is affecting all cylinders or it is a false misfire code. False misfire codes can be triggered by worn-out transmission, torque converter, or clutch. If all these seem to be OK, you probably have one of these components causing the misfire:
- Oxygen sensor – check data if it is switching
- EGR valve – blocked EGR can cause a misfire
- Vacuum leak – check fuel trims
- Timing issue
All of these conditions can be checked with the mode $01 – Current data, even on basic scan tools.
3. still not solved? Diagnose P0300
You solved the misfire on the specific cylinder, or you found and replaced the component you were sure was causing the issue. But the P0300 and misfire are still present in your vehicle. Or you just don’t know how to continue your fixing process? I was able to put together a few more suggestions that fixed the issue when mechanics felt stuck.
Perform compression tests
According to my misfire fix study, 23% of misfire cases were solved by either valve adjustment/replacement or whole engine rebuild. You can do a cranking compression test, but it may not detect engine head issues, only engine block problems.
Verify the correct operation of your engine head by running a compression test with your pressure gauge, or if possible with your bi-directional scan tool. Some scanners can do compression/cylinder power balance tests by turning off fuel injectors one by one on specific cylinders.
Change for OEM part
You were 100% sure that your diagnosis was right, you changed the faulty part, and maybe a few days passed and the misfire is back. And doesnt matter how many diagnostic tests are you doing, you cannot find anything else.
A lot of mechanics were in similar situations when dealing with misfires or other engine issues. The diagnosis pointed to an oxygen sensor for example, but changing it to the aftermarket one didn’t solve it. When the oxygen sensor was changed to OEM (original from the dealership) unit, suddenly the misfire is gone.
Test sensors
Not very common but I did find cases of misfire caused by one of the following sensors. Make sure they are working correctly.
- MAF (mass airflow) sensor
- TPS (throttle position) sensor
- CKP (crank position) sensor
- O2 (oxygen) sensor
Diagnostics Introduction!
- Automotive electrical basics
- OBD-II scanner basics
- Using cheap tools to diagnose faults
- Bi-directional scan tool introduction
- OBD-II scanner Buyer’s Guide
- Free diagnostics training
I test OBD-II scanners and make DIY Engine diagnostics guides to help you solve your car problems without having to depend on the mechanic. A lot of them will try to scam you or are just no help at all. About Juraj Lukacko