P0300 code with flashing SES light

signalnc

Original poster
Member
Dec 28, 2012
249
Hi. I'm new here to the forum. I have a 2003 trailblazer , 4.2, 4x4, ext LT with 128k on it. Not much has been done to the vehicle except oil changes, tires, and air filter changes.
I had the service engine soon light come on about a month ago and I ignored it. Then on Christmas day I put 20 dollars of 93 octane from a gas station that I had never been to before. I drove about 32 miles and parked the truck for 5 hours. then before leaving I went out to warm it up. It started fine and seemed to run fine. I went back in the house and came out 10 minutes later to find the truck had stalled out. I restarted it and started driving. I immdiately could feel it was running rough and misfiring. Then the service engine soon light started flashing. I made it home and parked it. the next day I cleaned the throttle body and changed the plugs, both of which had never been done before. I used autolite double platinum plugs because they didn't have delco's.
OK then I restarted the trailblazer and took it for a drive. The traiblazer is still running rough, and the check engine light came back on and it flashes when I accelerate. I took it to the auto parts store and they checked to see what codes it is throwing and it came back a P0300. I put some sea foam in the gas tank with about 7 gallons of gas, and I drove it about 25 miles and no change. I also plan to check the coil boots today. Is there anything else I can check? Thanks
 

Matt

Member
Dec 2, 2011
4,029
The P0300 code is for a random misfire, which typically indicates there's an issue with one of the coils. If the code is P030X, X (1-6) indicates the cylinder number. I would look at a coil being suspect because it started before you changed the plugs.

There's two ways that I know of to diagnose it...wait to see which cylinder number pops because it eventually will or buy a new coil and replace them, starting at the front cylinder until the rough idle goes away.

Also, as an aside, you'll really want to change all the fluids in the truck...coolant, transfer case, front and rear diffs and trans. They are all way over due and will hurt you in the long run if you don't do them.
 

signalnc

Original poster
Member
Dec 28, 2012
249
Thanks Matt. I'll get a new coil and change them one by one. I will also take your advice and change the fluids too.
 

dfc739

Member
Jul 29, 2012
170
Des Moines, IA
Stop driving the vehicle if you can. You're going to destroy your catalytic converter otherwise. The misfires are throwing unburned fuel at the converter and making it overheat and melt (that's why your SES light is flashing). Best case scenario is you'll have minimal damage. It could also melt some or most of your converter... which is rather expensive to fix.

Change your plugs to the AC Delco 41-103 Iridiums. I don't think it's going to be one of your coils. If it were one of your coils, it would throw a specific misfire for that cylinder (P0301 for cylinder #1 and so on). A P0300 is usually a general electrical problem or a vacuum leak. Check the wiring harness that powers the coils for any damage and check the battery. Also check that you don't have any vacuum leaks. The intake manifold is a common place to have the bolts come loose and leak. Check all of them (I know they are hard to get to) to make sure they are at least snug. If you have a mechanics stethoscope, you can use it to listen for vacuum leaks. Did you completely remove the throttle body to clean it? You might also check the hose connected at the throttle body to make sure it's attached properly. Did you disconnect the battery for 30 minutes after cleaning the throttle body?
 

signalnc

Original poster
Member
Dec 28, 2012
249
I took it back to the auto parts store to get the coil and had them check the codes again. this time it showed a P0302, so I replaced coil #2, started it up and it runs smooth. Going to take it for a ride in a little while.
 

Matt

Member
Dec 2, 2011
4,029
signalnc said:
took it out for a drive. It's running good now. Thanks for the help.

:thumbsup:

If you have an android phone, look at getting Torque and a cheap OBD II adapter so you can read the codes and do other cool stuff yourself.
 

acdear217

Member
Dec 23, 2012
3
My 2005 TB is doing the same thing. Got call from wife telling me ours was doing the same thing, came home from work hooked up my scanner and no codes found, let it run for 5 mins and on came the light giving me P0300. Grabbed the spare set of plugs I keep for it (ACDelco's), started back up and could still hear it missfiring, plugged scanner back in this time only reading P0305 for cylinder 5. Took coil back of thinking maybe I didnt get it seat all the way on the plug as its happened to me before. Retested and still the same code. Swapped cylinder's 2 and 5's coils to see if the misfire will follow the coil. Still showing cylinder 5, thought maybe it just needs a bit to re-adjust itself. Went down the street a total of 1.2miles with scanner hooked up as i was driving. Now reading P0300 again and running worse then before I swapped coils around and light flashing when over 30mph. To dark and cold to continue guessing outside with it ( 8degreesF) the entire time I've been out there doing this. I need some help and guidance, should I pick up a new coil and start setting it into different cylinder or start looking into putting in a new intake gasket. The throttle body was cleaned over the summer when I installed the K&N cold air kit and AirRaid spacer with JET module. I need help quick as this is our only family car and with 3 kids needing to be carted around cant afford any down time with it.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Welcome! You said you "grabbed" the spare plugs. I assume you installed them? Were they 41-103?

Don't drive too long with a flashing SES light - that's going to kill/clog your cat, costing more than a rental for a few days if you need downtime.

I'd try a coil. Got a buddy to swap with? If not, check compression and the tightness of the intake manifold bolts.
 

acdear217

Member
Dec 23, 2012
3
They were a set of brand new ones I bought last summer when i hit 100,000 miles. Yes they were AC Delco 41-103 iridium plugs and installed them. Trying not to drive it, as the problem just started today at about 3pm est, and they only driving done since then was about 2 miles including my 1.2 mile trip to the store. Dont have a buddy but have good connections at the parts store. I've been reading on this forum and others where people have had the same problem had their tbs at the dealer for over a month trying to figure out what the problem is with many misdiagnostics and unneccessary repairs ( replacing heads, exhaust valves, injectors, fuel rail, etc). My buddy brought to my attention that I never thought of was since it was -15 here last night and only got up to about 10 degrees that possibly ice crystals going through the injectors and getting stuck. I have to wait till the morning to go and get some dry gas and injector cleaner. was going to try that. Also read that its been as simple as a carbon build up and did a carbon clean and their problems were gone. I am just nervous that this will be a time consuming project to fix and I can not afford that, its our only family car and not much extra money we cant afford to spend the money for rentals or getting the dealers to look at it, I do all repairs myself to save money that way. I just dont want to spend the money on the coil and not have it be the problem, since when i switched the coild around the misfire didnt follow the coil rather re through the random misfire.

- - - Updated - - -

They were a set of brand new ones I bought last summer when i hit 100,000 miles. Yes they were AC Delco 41-103 iridium plugs and installed them. Trying not to drive it, as the problem just started today at about 3pm est, and they only driving done since then was about 2 miles including my 1.2 mile trip to the store. Dont have a buddy but have good connections at the parts store. I've been reading on this forum and others where people have had the same problem had their tbs at the dealer for over a month trying to figure out what the problem is with many misdiagnostics and unneccessary repairs ( replacing heads, exhaust valves, injectors, fuel rail, etc). My buddy brought to my attention that I never thought of was since it was -15 here last night and only got up to about 10 degrees that possibly ice crystals going through the injectors and getting stuck. I have to wait till the morning to go and get some dry gas and injector cleaner. was going to try that. Also read that its been as simple as a carbon build up and did a carbon clean and their problems were gone. I am just nervous that this will be a time consuming project to fix and I can not afford that, its our only family car and not much extra money we cant afford to spend the money for rentals or getting the dealers to look at it, I do all repairs myself to save money that way. I just dont want to spend the money on the coil and not have it be the problem, since when i switched the coild around the misfire didnt follow the coil rather re through the random misfire.
 

CaptainXL

Member
Dec 4, 2011
2,445
Other than a bad coil (which you proved was not the case) is the possiblity of a stuck fuel injector on #5 or a burnt exhaust valve.

I keep seeing this on the forum. Usually due to poor fuel sources. But not all the time.

My suggestion and what has worked well for me is to ensure you run some top tier fuel (87 octane) for a while/month and run some Techron Concentrate or BG 44K fuel injector cleaner through the tank.

The fact that you swapped #2 and #5 and still recorded misfires on #5 suggests to me that the injector is rich because it is sticking open at times thus fouling anything in the cylinder which includes valves and spark plug.

Don't be afraid to get it out on the highway everyonce in a while and get the rpm's up a bit. Floor it when getting on the highway.
 

acdear217

Member
Dec 23, 2012
3
Yes it still recorder the misfire but the misfire didnt follow the coil but reverted back to the original random misfire. Right now I run 87 octane from the same gas station since I bought it in 2007 and never had an issue with them ( this vehicle or any of my others in the past) And every oil change I run either seafoam or lucas injector cleaner in the tank, about every 2 to 3 months. I usually always romp on it (wife doesnt like it when i do lol) but with the light on and flashing over 30mph getting it onto the highway is not going to happen, dont wanna spend the extra $250 on a cat after it gets burnt up. I've got my buddy at the parts store going to bring me a new coil that I'm going to try just because it may be the simplest of things to do for now, and got a call in to a ase master tech certified in gm to try and squeeze me in to run some diagnostics on it if he has an opening ( will cost me some beer and coffee i hope). Will find out tomorrow if anything changes and I'm hoping for the best that there were just ice crystals sticking in the injectors. Never had a problem with this before and only had the typical tb troubles of weak water pump bearing and faulty sending unit.
 

CaptainXL

Member
Dec 4, 2011
2,445
acdear217 said:
Yes it still recorder the misfire but the misfire didnt follow the coil

I never said the misfire followed. There is a problem with something other than the coil as you noted in your previous post.

acdear217 said:
plugged scanner back in this time only reading P0305 for cylinder 5. Swapped cylinder's 2 and 5's coils to see if the misfire will follow the coil. Still showing cylinder 5
 

signalnc

Original poster
Member
Dec 28, 2012
249
did you clear the p0305 code after you swapped the coils? I remember after I cleared my codes out I got the p0300 code again, and then after driving it for like 25 minutes the p0302 code came on. I know that's bad for the cat to drive it with the flashing SES light, but that's just how it worked out. It seems it takes the computer a while before it pinpoints the bad cylinder.
 

CaptainXL

Member
Dec 4, 2011
2,445
In order to clear the codes you disconnect the battery for about 10 minutes.

Disconnecting the battery is strongly recommended anyway when working on anything electrical on the vehicle. Although not entirely neccessary for removing coils if the ignition is off ...its a good habit to get into.

We have had more than a few members unknowingly screw up some components.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
There were reports on the OS about broken wires at the coil connector. It may not be the problem, but it's something to check.
 

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