My check engine light has been on for about 4 months now, and is throwing a P0300 code. I changed both 02 sensors and the light is still on. The funny thing is that before I replaced the two sensors, the light would come on and off every now and again depending on the temperature outside. After I changed both 02 sensors, the light just stayed on. I have no idea what the issue may be. The car doesn't seem to lose any performance or major problems driving at all. One every few weeks it does misfire when in idle, but rarely. Does it just need a tune up? Is there something I should check out under the hood to see if it's worn out?

The P0300 code is for a random misfire. A tune-up might help but I doubt it. It never hurts to put a new set of plugs in.

However, random misfires are usually an indication that something else is wrong with the ignition. Something is happening between the PCM and the ignition to cause it to not fire on multiple cylinders. That is usually an indication that there is a bad ground, a loss of 12V+, a bad PCM, and/or a bad ASD relay.

I'm having similar issues with the misfires and my problem lies somewhere between the ASD relay and the PCM. I can eliminate all misfires by removing the ASD relay and installing a jumper wire that cost almost nothing. I'm not happy with that as a long term fix so I keep searching for an answer. It's been to 4 different dealerships and no one can figure out what the problem is. They have all come to the conclusion that the only way to drive my car is with the ASD relay jumpered out.

My check engine light has been on for about 4 months now, and is throwing a P0300 code. I changed both 02 sensors and the light is still on. The funny thing is that before I replaced the two sensors, the light would come on and off every now and again depending on the temperature outside. After I changed both 02 sensors, the light just stayed on. I have no idea what the issue may be. The car doesn't seem to lose any performance or major problems driving at all. One every few weeks it does misfire when in idle, but rarely. Does it just need a tune up? Is there something I should check out under the hood to see if it's worn out?

I was having random misfire code baught 2 02 sensors thinking and being told by dodge dealership only needing 2 then when having 02 sensors change come to find out there's 4 02 sensors. 2 pre cat and 2 post cat. check or change other 2 02 sensors

I would try some fuel system cleaner in the gas tank. You may have a little water or some other contaminant in the tank that is causing the random misfires. I have a Neon that started misfiring on cyl #4 (P0304 code). I started with the plugs, then wires, then coil but nothing fixed it. I finally ran some Seafoam system cleaner in a full tank of gas and haven't had the problem since. I think there was carbon buildup on the valves and piston tops that was causing the problem and the Seafoam cleaned it up.

Thanks...I think I'll start cheap with the cleaner, and then try the other two sensors. Fortunately the sensors are cheap. I just hope this doesn't turn into something serious...it should be simple considering the performance hasn't generally been affected other than some random misfiring. Just drives me nuts because I thought it was the bank 1 and bank 2 sensors this whole time.

Thanks...I think I'll start cheap with the cleaner, and then try the other two sensors. Fortunately the sensors are cheap. I just hope this doesn't turn into something serious...it should be simple considering the performance hasn't generally been affected other than some random misfiring. Just drives me nuts because I thought it was the bank 1 and bank 2 sensors this whole time.

Before you go spend $10.00 on a bottle of Sea Foam, you might consider doing a little diagnostics work.

Here's why I'm suggesting this: You have a P0300 code and not a P030X code with the X representing the cylinder that's misfiring. A random misfire will 99% of the time be caused by a breakdown in the ignition system (PCM). 1 way to test this is with a jumper wire to jump out the ASD relay. The ASD relay is the square black plastic relay that is the closest 1 to the alternator inside that black box on the passenger side inner fender. Unplug it and look at the bottom 5 prongs. They are numbered 85,86,87, 87A, & 30. You want to make a jumper wire using 2 crimp on spade terminals and some 14 gage wire. Plug the jumper into the #30 and #87 sockets.

Clear the codes and test drive. You will get a P0688 and a P0685 code. That is OK. Those will show up becaue nothing is connected to the 85 and 86 terminals. If no P0300 codes appear after installing the jumper you are waisting time and money on the fuel system. The problem is in the PCM and/or the ASD relay circuit.

It may turn out that the ASD relay is bad and it only cost $10.00 (same as the bottle of Sea Foam). If you are a DIY type you probably already have a piece of 14 or 16 gage wire and some spade terminals. It takes only 5 minutes to do this test and it will answer a lot of questions.

Thanks...I think I'll start cheap with the cleaner, and then try the other two sensors. Fortunately the sensors are cheap. I just hope this doesn't turn into something serious...it should be simple considering the performance hasn't generally been affected other than some random misfiring. Just drives me nuts because I thought it was the bank 1 and bank 2 sensors this whole time.

What engine do you have and how many miles on it ?

Also, are the plugs original ?

2009 SE, 68,000 miles....plugs are original.

Before you go spend $10.00 on a bottle of Sea Foam, you might consider doing a little diagnostics work.

Here's why I'm suggesting this: You have a P0300 code and not a P030X code with the X representing the cylinder that's misfiring. A random misfire will 99% of the time be caused by a breakdown in the ignition system (PCM). 1 way to test this is with a jumper wire to jump out the ASD relay. The ASD relay is the square black plastic relay that is the closest 1 to the alternator inside that black box on the passenger side inner fender. Unplug it and look at the bottom 5 prongs. They are numbered 85,86,87, 87A, & 30. You want to make a jumper wire using 2 crimp on spade terminals and some 14 gage wire. Plug the jumper into the #30 and #87 sockets.

Clear the codes and test drive. You will get a P0688 and a P0685 code. That is OK. Those will show up becaue nothing is connected to the 85 and 86 terminals. If no P0300 codes appear after installing the jumper you are waisting time and money on the fuel system. The problem is in the PCM and/or the ASD relay circuit.

It may turn out that the ASD relay is bad and it only cost $10.00 (same as the bottle of Sea Foam). If you are a DIY type you probably already have a piece of 14 or 16 gage wire and some spade terminals. It takes only 5 minutes to do this test and it will answer a lot of questions.



Thanks for your help, this makes a lot more sense to me. I'll let you know the results.

68k...get a tune up...

Thanks for your help, this makes a lot more sense to me. I'll let you know the results.

FYI, if you are lucky, the p0300 code is still there after you install the jumper wire. Drive the car for a day or 2 and see if the code is there for p0300. If it is gone, I'm afraid you have the dreaded ASD relay malfunction.

If so, replace the ASD relay and recheck. If your lucky, the new relay will solve your problem. If you're unlucky, the problem will persist and it will get worse.

I'm working very hard to resolve this. I have a malfunctioning ASD and I have already replaced everything except the wiring harness. I have a Chrysler Extended Waranty and they won't replace the wiring harness. I already paid for everything else. I priced a new wiring harness for the engine and it was $1,200.00. I was going to just buy a new one and do it myslef, but $1,200.00 is too much for me to experiment with right now.

Status update: after running all of the tests indicated, putting fuel injector cleaner, and replacing all of the spark plugs, I ended up having to take it to a dodge dealer to get a proper diagnostic test for the misfire. Turns out it was the perch valve. It was not opening or closing correctly so therefore throwing the p300 code since the fuel wasn't burning correctly. Glad I took it in before throwing more parts at it.

Status update: after running all of the tests indicated, putting fuel injector cleaner, and replacing all of the spark plugs, I ended up having to take it to a dodge dealer to get a proper diagnostic test for the misfire. Turns out it was the perch valve. It was not opening or closing correctly so therefore throwing the p300 code since the fuel wasn't burning correctly. Glad I took it in before throwing more parts at it.

Can you get a part number from the dealer? I'm curious as to what this is.

Yeah, what they said. What's the part number of the perch valve

Bump

What is the valve you are referencing? Can you provide a part number?

The Perch Valve?Is that next to the muffler bearing?never heard of a perch valve,like to hear this one.

Maybe he meant to put purge valve?

Maybe he meant to put purge valve?

I hope so

I hope the OP follows up with this. It's the only way we can solve this misfire/stumbling and/or stalling problem.

Ok so it has been almost a year now, and I have been driving around with this annoying check engline light on. I have tried everything under the sun.....fuel injection cleaner, a tune up, all new spark plugs, and replaced all four 02 sensors. Re-set the battery and the damn light still came on. Like I said though, I have not had any issues with performance or anything but it still bothers me as to why it keeps throwing a p0138 code now after all of this. What could it possibly be? I figured it couldn't be serious if I have been driving like this without any issues for a while now, but I really want to figure out what is going on.

And yes, I meant purge valve earlier....sorry I didn't clarify. Later on the dealership suggested I replace the whole gas tank....claiming that I can't just fix the purge valve. I ended up replacing just the purge valve and it's still throwing a code......it honestly can't be the gas tank can it???

Do you have a 3.5L? There is a TSB out for the 3.5L with P0300.

Fuel pump? Fuel pressure regulator?

Do you have a 3.5L? There is a TSB out for the 3.5L with P0300.

2.7. Would it also apply to that model? I don't think it's the fuel pump....no performance affect at all, no misfires. Just the code with no symptoms.

I know this is an older thread but I recently found my way here while searching the web for possible causes and fixes to the P0300 and P0302 engine codes I was seeing on my '06 Charger (3.5L V6). I replaced the plugs and gave it a good dose of Seafoam - nothing changed. Then I tried swapping the #2 and #4 coils to see if I had a bad coil - still got the P0302. Tried swapping plugs in case one was bad . . . no change. Finally I decided to move to the ASD relay test. I jumpered terminals 30 and 87 as explained in this thread and initially got the P0685 and P0688 codes. Then I tried to drive my car and all kinds of crap went wrong. It started to chug and sputter and ran like crap. Next thing I know I've got 4 new codes - P0031, P0037, P0051, P0057 - all related to the heater circuit in my O2 sensors.

I'm not sure the issue was the result of my jumpering the terminals but as soon as I pulled the jumper and replaced the relay the car ran fine. All I know is everything went south after I jumpered the two terminals. Since then I haven't been able to clear the new O2 sensor codes. I took the car to my mechanic this morning and was told my PCM is fried . . . that's around $750 for the part and programming. He thinks my jumping the terminals together likely shorted out the O2 heater circuit in the PCM since power for the heaters comes directly from the PCM and they are no longer getting any power, so I'm off to the dealership to order a new PCM and then get them to program and install it.

As for the original P0300 code I've been told by both my mechanic and my local dealership that 99% of the time it is the result of carbon build up on the valves, and that Seafoam and similar products may not be enough to remove it. The valves in the 3.5 spin to clean carbon, but the system doesn't work that well apparently. There's a fix but it involves replacing components in the heads . . . no thanks! Option 2 - run higher octane fuel which has a cleaning additive (and works well with my Diablo inTune!).

Just wanted to post my experiences in case anyone else finds their way to this post like I did. Personally I don't recommend the ASD relay experiment - better to just buy a new one for $10 - $15 if you suspect it is the problem! Maybe my PCM failure had nothing to do with my test of the ASD but I'll probably never know . . .

Thank you for sharing your experience! I suspect that my issue is the same and that I need to purchase the part and reprogram. The fact I have tried everything under the sun (replaced all sensors, replaced coil and spark plug for bank 1 and 2, got a tune up, checked the purge valve per dealership recommendation, etc etc) and the car still runs fine without any symptoms or issues..... it has to be a programming issue. It's just annoying to have had that check engine light on this whole time and not know what the hell was causing it after all of the diagnosing and troubleshooting I have done in the past three years i've had the car. I guess it could be worse. The car still runs perfectly fine, and I'll just have an issue if I try to trade it in with the light on. Still deciding whether it's worth it for me to buy and reprogram the PCM.....i'm afraid it won't fix the problem and the light will still be on!

PCM's are usually warranty-covered for 8yr/80k miles, federal warranty, but you need to check with a dealer first and they have to be the ones doing the diag/repairs

I know this is an older thread but I recently found my way here while searching the web for possible causes and fixes to the P0300 and P0302 engine codes I was seeing on my '06 Charger (3.5L V6). I replaced the plugs and gave it a good dose of Seafoam - nothing changed. Then I tried swapping the #2 and #4 coils to see if I had a bad coil - still got the P0302. Tried swapping plugs in case one was bad . . . no change. Finally I decided to move to the ASD relay test. I jumpered terminals 30 and 87 as explained in this thread and initially got the P0685 and P0688 codes. Then I tried to drive my car and all kinds of crap went wrong. It started to chug and sputter and ran like crap. Next thing I know I've got 4 new codes - P0031, P0037, P0051, P0057 - all related to the heater circuit in my O2 sensors.

I'm not sure the issue was the result of my jumpering the terminals but as soon as I pulled the jumper and replaced the relay the car ran fine. All I know is everything went south after I jumpered the two terminals. Since then I haven't been able to clear the new O2 sensor codes. I took the car to my mechanic this morning and was told my PCM is fried . . . that's around $750 for the part and programming. He thinks my jumping the terminals together likely shorted out the O2 heater circuit in the PCM since power for the heaters comes directly from the PCM and they are no longer getting any power, so I'm off to the dealership to order a new PCM and then get them to program and install it.

As for the original P0300 code I've been told by both my mechanic and my local dealership that 99% of the time it is the result of carbon build up on the valves, and that Seafoam and similar products may not be enough to remove it. The valves in the 3.5 spin to clean carbon, but the system doesn't work that well apparently. There's a fix but it involves replacing components in the heads . . . no thanks! Option 2 - run higher octane fuel which has a cleaning additive (and works well with my Diablo inTune!).

Just wanted to post my experiences in case anyone else finds their way to this post like I did. Personally I don't recommend the ASD relay experiment - better to just buy a new one for $10 - $15 if you suspect it is the problem! Maybe my PCM failure had nothing to do with my test of the ASD but I'll probably never know . . .

Shimmy,

Terminal 30 is hot (12V+) from the battery and terminal 87 goes to all 8 injectors and ignition coils. When you jumper those 2 terminals you send 12V+ to the coils and injectors (nothing else).

The exact same thing occurs when the PCM connects terminal 86 to ground. The relay energizes and closes the contacts which connects terminals 30 and 87.

Jumping across the contacts of a relay is a standard procedure for testing to see if the relay and/or the control circuit is bad. It's done a thousand times a day in every dealership across the nation.

Now, if you connected the 30 to 86 it would fry the PCM. If you connected 30 to 85 nothing would happen because both are 12V+.

I don't think you actually connected 30 and 86. I think your PCM was already going out.

Besides, this test is more for stalling problems than miss fires. If all you have is a miss then the problem probably isn't the ASD circuit.

The P0685 and P0688 codes are par for the coarse when the ASD is jumpered out. The PCM closes the ground circuit when it energizes the coil. It then sense a load (current draw). If the relay is pulled out it can't close the circuit that runs through the 85 and 86 terminals. It stores a code for this but there is no CEL.

Anyway, October of this year will make 3 years mine has been jumpered out.

I'd be glad to shoot a video of the car running with the jumper installed and then I will get in the car and drive it with my camera backed off from the wheel far enough to show there are no CEL's and the car runs like a scalded dog! I will pop the hood, get out, and then show the ASD jumper still installed and I will U-TUBE it.

I'm not trying to piss anyone off, but I do want to prove this. Chrysler Engineering even is aware of my situation. In the 6 months my dealer and Chrysler collaborated to try and fix my car they finally said GOODBYE! They have washed their hands of it even though I have a 7year-unlimited miles warranty! They did have concerns over the ASD jumpering, but they never said anything about frying the PCM. The reason is quite simple. Once terminals 30 and 87 are jumpered the PCM is completely taken out of the circuit.

I will return with a Utube link.