2002 Hyundai Elantra 2.0, #1 & 2 cylinders misfires, spark plugs are good, compression 145 PSI, #1 & 2 Injector not firing, I switched injectors with 3 & 4, & they are working. Vehicle is monitered by the DUI system in Florida.

Check these three things in the misfiring cylinder: spark, gas, and compression. One of them is wrong or missing. For spark check the spark plug and wire. Make sure you have spark on the plug wire, a strong blue spark. For gas, the fuel/air mixture may be wrong. Does the fuel injector pulse, putting gas into the cylinder? Possibly you may have a leaking injector or a clogged injector.

If gas and spark seem okay, you need to check compression in the cylinder. Low compression will cause a misfire. If compression is 20 or 25% lower than adjacent cylinders, that could be causing a misfire. For example, say numbers 1 and 3 cylinders had about 150 psi compression. 80% of 150 = 120, so if cylinder 2 is 120psi or less, that could be setting the misfire code. Low compression can be caused by valves not seating well, or bad piston rings, bad head gasket- something in the combustion area of the cylinder is losing compression.

Yes it does. A fuel injector is a device that atomizes and then delivers the fuel into the cylinder just before the piston squeezes the fuel and air mixture and the spark from the spark plug fires. Ergo your 2000 Hyundai Elantra being a 4 cylinder vehicle, has 4 fuel injectors.

Is it misfiring on just one cylinder (code 0301, 0302, 0303, 0304 etc.) or multiple/random (code 0300) cylinders?
It takes all 4 ingredients to fire a cylinder - Air, Fuel, Spark, and Timing. If you only have one misfire code (P0302 - Cylinder #2 Misfire), it should be a relatively easy diagnosis and fix. Let's start with Spark. I'm going to assume you have a code reader. First, remove the spark plug in #2 cylinder. Examine the threaded end. Is it wet and/or smell like fuel? If so, chances are the misfire is caused by a weak/absent spark. To confirm, swap it with a known good cylinder - example #3 cylinder. Clear the code. Start it and let it run for a couple of minutes. If a code appears with P0303 (the last digit refers to cylinder number), then you have found the culprit - bad spark plug. If the code reappeared with P0302 (Cylinder #2 again), then swap out the spark plug wire (both ends). Clear the code. Start and run for a couple of minutes. Again, if it appears on Cylinder #3 - the plug wire is the culprit. If the P0302 code comes back, swap the coil. See where this is going?
Now, when you first pulled the plug from Cylinder #2, if it wasn't wet or smelled of fuel, chances are Spark is not the issue. Was the plug dry and/or ashy? Possible cause is no fuel reaching the combustion chamber (cylinder). Using an automotive stethoscope, listen to the fuel injector for cylinder #2 while the motor is running. Should hear a regular and rhythmic ticking sound. Like a fast clock. (Use a long handled screwdriver touching one end to your ear and the other to the top of the injector if you dont have a stethoscope). No clicking or irregular clicking means fuel injector is not working (no sound) or is not working properly. Like searching for the Spark problem, swap the injector in #2 cylinder with #3. Clear code. Start and run. If the problem moved to #3, ithat injector is the culprit. If it stayed on #2, it is likely your wiring to that injector - check the whole harness for loose connections or swelled with oil wires/connectors. Keep pecking away at the fuel delivery system for that cylinder. Stay looking for things that affect only that cylinder.
When you pulled the plug, was it fairly normal looking? Then this misfire code might only be a timing problem. It is likely not a timing problem in your case, because timing affects all cylinders, but I thought I would just mention it since Timing is an important part of combustion. Also, if timing were a culprit here, another code would usually appear signifying the Camshaft Positon Sensor or Crankshaft Position Sensor was/is failed, or it is possible for the timing belt/chain to skip a knurl on a sproket and throw the timing off, but like I said, it would almost always cause multiple/random (Code P0300) misfires.
And finally, when you pulled the plug earlier, if it was an oily sticky mess, then the culprit might be Air - or compression/lack of compression. Lets hope not, because when Air is the cause of a misfire, it is usually an expensive fix. Need to perform a compression test. If you have little or no compression in the cylinder, it is likely a bad head gasket or a stuck/sticky/broken/chipped valve (exhaust or intake). Any of these things are bad news and expensive to fix in a shop. Also bad news and time consuming and difficult task for a shade tree mechanic.
Now, clear that code and get started! Good luck!

Misfires are caused by one of 3 things-gas, spark, or compression-one of those is the problem. For gas, check the fuel pressure and see if it is at the right pressure for your car. Low fuel pressure will cause misfires. If pressure is good, it could still be an injector problem, like a clogged injector. If low pressure, suspect the fuel pump or the fuel filter.
For an ignition spark problem, check your spark plugs. Pull them out and inspect them for broken or cracked plugs, check the spark plug gap, check for oil fouled plugs. You may be able to isolate the misfire to a cylinder or two by the plugs condition. A misfiring plug or cylinder should show more black, sooty deposits than the other plugs. A normal firing plug will have a tan or brown colored insulator on the end with very little deposits. If the plugs are okay, clean them a bit if needed, and check for spark on the spark plug wire ends with engine cranking. Buy an inexpensive spark tester, or hold each spark plug wire metal end terminal 1/4 inch from a ground on the engine block, while cranking the engine. Wear a heavy glove to avoid getting shocked. Check that every cylinder is getting a strong blue spark to eliminate ignition as a problem. A weak, yellow or orange spark is an ignition problem, possibly a bad coil, or other ignition part.

If spark and gas is good, you may need to check engine compression. Any cylinder that has significantly lower compression than the other cylinders will cause a misfire, and set a misfire code. You should check for any trouble codes in the engine computer. A code reader or scanner would have to be connected to the diagnostic connector under the dash. What is "significantly lower" compression? About 15-20% less compression. Say you checked and had 150 psi compression in most cylinders. If one had 20% lower compression ( 150 X 20%= 30 psi, 150 -30 =120 psi, or same as saying 150 X 80%= 120 psi.), a condition like that can cause a misfire. I hope I have made sense here. Good luck, and post back what you find, or if any questions.

Hi Richard, I'm glad to help! Richard, there's a lot of things that can cause one certain cylinder to misfire. I would first: "Replace the Spark Plug and Spark Plug Wire for number 4 and see if the misfire goes away. If it doesn't then you need to do a "Compression Test" for cylinder # 4 and compare it to another cylinder and if its more then 25 Psi difference then you need to do a "Cylinder Leak Down Test" to see where you are loosing the compression. If the compression is good and you have new plug and wire and it still misfires. Then you'll need to check and make sure you have good spark coming from the coil. If you have good spark coming from the coil. Then you'll need to check and make sure the # 4 Injector is firing. Hope this helps and have an awesome day Richard.

If you pull the #2 plug out and start the engine do you see a spark? You didn't mention if you checked the wires or not, but it could be a bad plug wire. If there is a spark, I'd try and see if the #2 injector is actually firing like you mentioned above.

are the leeds ok and the coils this also causes a miss fire

P0301 - Cylinder 1 Misfire Detected.

Possible Causes:
- Improper spark plug at cylinder #1
- Insufficient compression at cylinder #1
- Incorrect fuel pressure
- The injector circuit is open or shorted at cylinder #1
- Fuel injector #1 may be faulty
- Intake air leak at cylinder

Possible Solutions:
- Perform Injector Service
- Replaced Spark Plug #1
- Replaced Injector #1
- Replaced Ignition Coil #1

Being you have a heavy gas smell I would look into the injectors. If injector #1 is over producing it can cause a misfire. Test your compression and fuel pressure.

replace spark plug #2 and plug wire if you still get misfire you have faulty injector or bad valve or compression rings on #2 cylinder.

First check plug and make sure the right plug for this vehicle,make sure that the heat rating for this plug is correct. Your problem could be a wrong heat rating plug been installed. Also make sure that the misfire is due to a bad plug and NOT a bad injector or injector wiring or uneven compression or even bad coil. Check injector wiring and make sure it not shorting or broken. To check injector pulse use a NOID light can be bought at almost any auto parts store like Auto-zone etc. Also compression can be checked with compression guage. A rule of thumb to go by is if any cylinder has more than 10 PSI in difference in compression there is a problem like a burnt valv etc. Here is a n example cyl# 1 170psi cyl#2 176 psi cyl#172psi cyl# 4 150 psi, there is a problem with cylinder # 4 and will cause a miss from cylinder #4. Good luck and thank for using FIX YA