P0301 - Cylinder 1 Misfire Detected
spark plugs can tell a lot. Signs of arcing on the ignition coil, distributor cap or rotor can also point to a faulty part. Basics, such as the compression, timing and fuel pressure are also may need to be checked.
A faulty on-plug ignition coil is typically identified by swapping the coils between the cylinders and checking if the misfire moved with the coil or stayed at the same cylinder.
The engine might need to be checked for vacuum leaks. Read more: Vacuum leaks: common sources, symptoms, repairs. The short term fuel trim (STFT) and long term fuel trim (LTFT) need to be checked with a scan tool to see if the air/fuel mixture is not too lean. Often a bad mass air flow sensor causes the engine to run lean causing random misfire. Read more about the STFT and LTFT fuel trims here.
Pressure-testing the cooling system might help identify a leaking head gasket.
Valve adjustment may need to be checked, as valves that are out of adjustment can cause misfiring at a cold start, which is, a fairly common issue in some older Honda engines.
If misfire happens only at idle, the EGR valve and purge valve need to be checked, as either one could be stuck open.
Faulty mechanical valve train components like worn camshaft lobes or stretched timing chain can also cause the engine to misfire; they too might need to be inspected carefully.
Sometimes a broken or damaged tooth at the crankshaft or camshaft sensor gear can be the source of misfiring; checking the sensor signal with an oscilloscope can help if crankshaft position sensor (CKP) or camshaft position sensor (CMP) are suspected.
Once the problem is repaired or no longer detected, the misfiring code will clear itself after some driving.
The repair depends on the cause. The key is to have the problem diagnosed properly. Even if you planning on doing repairs yourself, it's not a bad idea to have the vehicle properly diagnosed at a dealer or reputable repair facility, where technicians have access to up-to-date information, technical service bulletins (TSB) and advanced scan tool.
How much can a repair cause? It depends on the cause. For example, if the misfire is caused by a bad on-plug ignition coil, which is quite common on many newer cars, the repair might involve replacing a bad coil and all the spark plugs. This could cost $300-$400 for a 4-cylinder engine or $450-$700 for a V6.
Additional trouble codes can shed some light on the cause
Additional OBDII codes that are present along with misfire codes can shed some light on the cause. For example, if you have the P0300 code along with P0171 - System too lean, there is a good chance that the misfire is caused by the lean air/fuel mixture that initially triggered the code P0171.
Once the problem caused the code P0171 is fixed, the misfiring will likely go away too. Another example, if you have the code P0401 - EGR insufficient flow along with the P0300, the EGR (Exhaust Gas Recirculation) system needs to be checked first.
Can a tune-up fix a misfire?
The term 'tune-up' usually means changing the spark plugs, ignition wires, distributor cap and rotor, and possibly replacing air and fuel filters. Newer cars need a tune-up less frequently, as most modern cars don't have the ignition wires or distributor and use long-lasting spark plugs.
If it's been a long time since you had a tune-up done last time, it's a good way to start; at least you will eliminate spark plugs and other tune-up items as a possible cause for misfiring. A tune up needs to be done at least every 60,000 miles anyway. It's best to use only OEM spark plugs. Read more about a tune-up here: How to maintain your engine - scroll down to the bottom.
Ignition coil: problems, when to replace, repair costs
Code P0171 - System Too Lean
How to maintain your engine: tune-up
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