At some point all 2000+ TJ 4.0 owners will have a misfire CEL. They can be very difficult to troubleshoot and expensive to just throw parts at. Mine was a cylinder #1 misfire, but this would work with any single cylinder misfire. Random misfires (P0300) would be troubleshot differently. I hope this helps others. Note: A P0303 may be caused by a Heat Soak issued which Jeep put out TSB 18-031-03 on it. Basically all you need to do is wrap a piece of insulation around the #3 injector and zip tie it on. The Jeep TSB calls for this product to fix the problem: https://www.amazon.com/gp/product/B007O3QHDK/?tag=wranglerorg-20 ********************************* I've been having a p0301 misfire for almost two tears. Never random or any other cylinder. But since my California registration is due and this is my year to pass smog, I figured that I better work on it ... I had until October, 20 to get everything good. Here's how I troubleshot and solved the issue: 1) Since this is a waste spark system (2 cylinders fire at the same time) and only cylinder #1 ever failed, I ruled out the Coil pack. I guess it could have been the #1 boot, but it looked good. 2) I pulled the spark plugs from #1 & 2 (#1 looked good), checked the gap and swapped them. Drove the Jeep and after about 150 miles the code popped again. It had a pending misfire at about 50 miles, but at around 150 it went hard. Checked the monitors and Catalyst and Evap were not ready. Note: In California, the Evap monitor does not need to be ready to pass the smog test. 3) Swapped injectors #1 and #2. Drove Jeep with same result as above. 4) Used a NOID Light to check injector signal to rule out the computer and injector wiring. All good. 5) Performed a dry compression tests on all cylinders, #1 was 125, about 10 below the next lowest, #3 ... A wet test yielded about the same results. Not enough for me to call it a ring problem. 6) Performed a leak down test to all cylinders, #1 leaked just a hair faster than the others, again not enough to call it bad. 7) Replaced both valve springs for cylinder #1. Drove Jeep with same result as above. 8) Replaced the intake/exhaust manifold gasket, maybe it had a leak I wasn't detecting. Note: these are MUCH MUCH easier to change if you spend 30 minutes and remove the front fender. Drove Jeep with same results as above. Analyzed past troubleshooting and determined that the problem must be in the head ... valve or head gasket. Removed the cylinder head and noticed a small burn on the cylinder #1 exhaust valve. My guess would be that once the valve rotates onto that area it reports a pending misfire code and once it rotates there again it pops a hard code. Since I had the 0331 cylinder head that was prone to cracking, I decided to replace it with a re-manufactured TUPY head. Total job completed by myself --- about 6 hours. Drove Jeep for 10 miles and had ALL monitors (except Evap) went ready. Immediately went to the smog shop where it passed with flying colors. Yeah!!! Since the successful smog test I have driven about 2000 miles with ZERO problems. In fact the engine runs much smoother now. Relief!!!