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1999 Honda Accord EX Misfire Diagnosis: P0300-P0304 and P1399 Codes Explained

Model: 99 Accord Fault Code: P0300 Posted: 2009-02-10 10:29

I've reviewed numerous posts on this topic and tried various troubleshooting steps, but the issue persists. Vehicle: 1999 Honda Accord EX (standard model), 125,000 miles, regular maintenance with no major issues. The car has performed well until recently. On Sunday, my engine began running roughly and the check engine light started flashing. OBD-II diagnostics show codes P0300 (random/multiple misfire), P0301-P0304 (individual cylinder misfires), and P1399 (manufacturer-controlled ignition system or misfire). Following forum advice, I tested each spark plug by unplugging it individually. All plugs appeared functional—removing any one caused a worse performance, indicating no single faulty coil. The engine runs smoothly at idle in neutral but becomes unstable under load. It runs poorly until around 2800 RPM, then suddenly jumps to 3000 RPM and stabilizes. When the transmission shifts and RPMs drop below 3000, it again runs rough until reaching approximately 2800 RPM. Yesterday I checked fuel trim values as recommended: 1. I/M Status: Catalyst, Evap System, O2 Sensor, EGR System — Not Ready 2. Short-term fuel trim fluctuated widely, mostly within ±5%, but spiked to +20% during the worst misfire episodes—just before the engine stabilized. My analysis (based on limited knowledge of fuel systems): - The issue may stem from insufficient air or fuel supply, possibly due to a clogged air filter or fuel filter. - A single cylinder misfire is unlikely since all cylinders are affected. However, if one plug fails early, it could place extra load on others, causing secondary misfires. - A faulty oxygen sensor might be skewing the ECU’s fuel calculation. Additional context: I was driving in mountainous terrain in West Virginia over the weekend—warm and wet conditions. The vehicle was lightly loaded, and my passenger was somewhat disruptive. Sam Cooke was playing on the stereo. Any insights or solutions would be greatly appreciated. Cheers, Ash

Related fault codes
P0300P0301P0302P0303P1399
Comments (10)
Anonymous 2009-02-14 21:24

How long have your spark plugs and spark plug wires been in place? This is a common factor when diagnosing P0300 and P0304 misfire codes on the 1999 Honda Accord.

Anonymous 2009-02-15 00:31

The spark plugs and wires have not been replaced since around 100,000 miles. I’ll need to check my maintenance records for exact dates. -Ashley

Anonymous 2009-02-15 21:28

That’s a reasonable timeline—older spark plugs can contribute to misfires even if not visibly worn.

Anonymous 2009-10-19 17:42

Hi, I’m experiencing P0302 and P1399 codes on my 1999 Honda Accord. Any advice or solutions you’ve found would be very helpful.

Anonymous 2009-10-20 05:03

On most Honda engines, widespread misfires like this often point to ignition components—such as a single coil or distributor connector—or EGR system issues with clogged ports. It's also important to check fuel pressure and verify timing offset using an OBD-II scanner. Was the +20% fuel trim reading taken at idle? How does the oxygen sensor behave during operation? After stabilization, what happened to the fuel trim values?

Anonymous 2009-10-22 10:21

The issue was resolved by replacing all spark plugs. While I initially thought that since all cylinders showed misfire codes, it couldn’t be plug-related, a friend explained that when multiple plugs are near end-of-life, one failing can overload the others, causing secondary misfires. Though I didn’t fully believe this theory at first, changing the plugs fixed the problem—lesson learned: always start with simple and affordable fixes before moving to complex diagnostics. Thanks for all the helpful tips—I gained valuable insight into how my engine operates.

Anonymous 2009-10-22 19:10

Well done! This is a great case study on troubleshooting misfire issues in older Honda models.

Anonymous 2009-10-25 23:38

I didn’t have success with new spark plugs. The problem continued, and I eventually took it to my mechanic. A leakdown test revealed cylinder #2 had a burnt valve—diagnosis cost $1,400. The mechanic noted that Honda recommends valve adjustment every 105,000 miles or when noise occurs. Valves tend to tighten over time and only make noise when loosening. This highlights the importance of proactive maintenance on older Honda engines.

Anonymous 2009-10-26 00:02

That’s valuable insight—after I adjusted my valves a few months ago, I found it's actually not that difficult once you understand the process and tools involved.

Anonymous 2014-03-08 17:29

I had exactly the same issue! The fix was a loose spark plug. I replaced all plugs and wires as a precaution—now everything runs smoothly again. Thanks for this post—it's incredibly helpful! Problem resolved: P1399, P0300, P0301, P0302, P0304