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Discussion Starter · #1 ·
I went to take my car in for my emissions test and it failed because the CEL was on. This is not surprising to me because the light has been on now for months. What is surprising is that I am getting a new code this time around.

Both the codes are:

P0304 - Cylinder 4 misfire detected (this is the one I've known about)
&
P0121 - Throttle/Petal position sensor/Switch A circuit range/Performance problem - This is the new one

The second one automatically makes me think it's the TPS but I know better to throw any more parts at it at this point. Any suggestions?

Btw, the car is a 2003 Hyundai Accent (1.6)

Thanks in advance for the tips (if I get any)
 

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Discussion Starter · #2 ·
The car idles badly and there is an obvious misfire that can be felt. When the car idles, the CEL blinks at me and then remains solid once I increase the RPMs. The cylinder 4 misfire code has been around for months and won't go away. I have changed the Plugs and wires in all cylinders, replaced the Timing belt, the PCV valve and the fuel injector for cylinder 4 (twice). I have tried spraying carb cleaner around the throttle body and the upper intake with noticeable change. I have trying to figure this out for months. Took it into the shop only to have them say "we don't know what's wrong with it"
 

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sounds like you've learned your lesson about throwing parts at the car. might as well toss darts at it.

yes the TPS is quite possibly the problem but explore air leak first (see below). i've personally found the ECU diagnostics to be flaky at best with these cars. its not terrible, just twitchy at times. doesn't always want to report the right fault or may miss a fault.

with that in mind, an air leak in the intake system can cause this problem. it would explain the misfires for sure and can cause the TPS to malfunction throwing the code. its actually a mismatch between the MAP sensor and the TPS...whatever.

i believe you may have a broken gasket on the intake manifold. unless you know what to look for, this can be a devil to figure out. had this on my accent as well. the 1.6L has a faulty gasket from factory and was later revised. the good news is that its easy to fix and the part was $32 from the stealership for me :)

testing isn't hard either. if you have access to welding gas, spray some argon around where the intake manifold mates to the engine head. the idle will smooth out if you have a leak. propane is another thought, a small unlit torch might help. that would cause the idle to increase.

another option is to use a can of the starting fluid or WD-40. same idea, idle should pick up. do note that gas tests are more accurate. argon is your best bet if you can get your hands on it due to safety. argon don't burn so well.

with that in mind, its also possible that a faulty TPS is the cause of your headaches. i somewhat doubt it since the misfires have gone on for so long with no TPS code - assuming they are related which they may not be. check for the gasket leak, then diagnose for bad TPS as necessary. i'll PM you some info regarding TPS testing.
 

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Hi Guys, just sorted out my same problem, had mis-fire stutter especially under heavy throttle, I changed spark plugs & wires & didn't make a difference which bummed me out as I know it gets more complicated & expensive from here. Tested sensors that may cause the problem first being TPS then CPS & then MAP & Idle sensor all 100% next was fuel system, changed fuel filter then did a fuel pressure test which came out 10-25psi way too low so I ripped the pump & bench tested it @ 80psi without strainer so I checked the pump strainer & it was gunked up inside, bought a replacement & now my car is running 100% actually better than ever much pokier on take off & NO MISSING :) hope this helps.
 
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