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Old 11-14-2017, 05:56 PM   #1
Jon Boy
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Default P0340 code

Yesterday on the way to work, my '01 SC2 started running rough and flashing the SES light. I managed to limp to work and used a scan tool to check the codes. It said there was a P0300 and P0341. I got it towed home and started going through the diagnostic flowchart here.

I checked the plugs, and they were looking pretty bad—the gap on one of them was about .072. The resistance on the plug wires was a little low too, so I replaced both the plugs and the wires. I did a little test drive around the block, and it seemed to be running fine again. I went to go to work, and suddenly it was running rough and flashing the SES again. This time it says it's a P0340.

I've checked the plug wires and towers for corrosion, and everything looked good. I checked the resistance from wire 1 to wire 4 and wire 2 to wire 3, and I also checked the resistance on each wire individually and on each coil pack. Everything is within spec. I also checked for arcing and didn't see any.

I backprobed circuit 633 at the PCM, and it was at 4.95 both with the ignition on and the engine on. I also checked the voltage on that circuit at the ICM end, and it was the same.

Everything seems to be connected securely—each plug clicked on each end when I plugged it in. The wiring order at the coil packs is 4-1-2-3.

As far as I can tell, I've been all the way through the diagnostic flowchart, and everything checks out. But it's still running like crap, spewing really rich exhaust, and throwing a P0340 code.

I'm kind of at my wit's end. Where do I go from here?

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Old 11-14-2017, 09:36 PM   #2
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Default Re: P0340 code

Well, I finally figured it out. When I got the car home last night, I swapped out the ICM and coil packs for an extra set I had lying around. (I replaced them trying to fix another problem, but that turned out not to be the problem.) This morning, I replaced the plugs and wires. Then I test drove it, and it worked.

But then, because both sets of coil packs tested within spec, I put the newer coil packs back on, and that's when it started saying P0340 rather than P0341. It wasn't until I looked at the data from my scan tool and saw that it was misfiring on 1 and 4 that I swapped out the coil pack again, and now it works.

The weird thing is that the faulty coil pack still tests within spec—it's right at 8K ohms resistance. So I'm not sure what's wrong with it, but at least my car's running fine now.

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Old 11-15-2017, 12:45 PM   #3
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Default Re: P0340 code

I guess I spoke too soon. It's misfiring on 1 and 4 and giving me a P0340 code again this morning. I swapped the coil pack again, along with the ICM, and it's still misfiring on 1 and 4.

Does anyone know what could cause an ignition misfire on cylinders 1 and 4 even when the coil pack and ICM have been replaced?

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Old 11-16-2017, 12:43 PM   #4
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Default Re: P0340 code

Anybody?

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Old 11-16-2017, 01:48 PM   #5
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Default Re: P0340 code

Sounds like it's time to replace the PCM, though there is a test or two more you can do (attached). Also a great time to do a compression test, cheaper to rent a compression tool and verify the engine doesn't need a rebuild than replace the PCM.
Attached Images
File Type: jpg P0340_Page_1.jpg (105.3 KB, 10 views)
File Type: jpg P0340_Page_2.jpg (143.5 KB, 4 views)

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Old 11-16-2017, 02:33 PM   #6
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Default Re: P0340 code

I tested the compression, and it was around 180 on all cylinders. I think the lowest was 178.

The cylinder 1 and 4 misfire counts are definitely filling up, so maybe I should jump over to the P0300 diagnostic flowchart. Do you happen to have that one too?

Also, what does LAST TEST mean on that flowchart?

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Old 11-16-2017, 02:51 PM   #7
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Default Re: P0340 code

Oh, and I forgot to mention that I've gotten a U1301 code a couple of times. I have no idea if that's related.

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Old 11-17-2017, 01:10 AM   #8
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Default Re: P0340 code

It's a FSM document, it assumes the user has a Tech II so I assume it's one of the tests you can run using said Tech II. Knock-offs/alternatives may do the same test just under a different name. What ever tool you have, to properly do the diagnostics it needs to be able to read "CMP Resync Counter" and monitor which cylinder is misfiring.

U1301 indicates a voltage issue with the data link between the PCM, BCM and ABS control module. Nothing directly to do with a misfire (but it hints at an issue with those modules, one of which could cause misfires if it's bad).

P0340 indicates a voltage issue with the ICM to the PCM (yes, it's more complicated than that but basically thats what it is)

P0300 indicates something other than a voltage issue, petty much anything from a vacuum leak to water in the fuel.

1st 3 attached pages are P0300, the rest are U1301.
Attached Images
File Type: jpg 01.jpg (50.3 KB, 8 views)
File Type: jpg 02.jpg (32.1 KB, 6 views)
File Type: jpg 03.jpg (174.4 KB, 6 views)
File Type: jpg 04.jpg (141.2 KB, 6 views)
File Type: jpg 05.jpg (52.3 KB, 5 views)

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Old 11-17-2017, 01:38 PM   #9
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Default Re: P0340 code

Thanks. My scanner is an OBDLink SX, and I bought the professional add-on for the OBDwiz software. I don't remember if it has that feature, but I'll poke around and see what I can find.

It looks like there's a junkyard in the area with a PCM from a third-gen DOHC with a manual transmission, so I might go grab it and swap it to see if that helps. Do I just need to worry about the engine and transmission, or do I need to check to make sure that the donor car has all the same options? I don't need to have the PCM reprogrammed or anything, right?

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Old 11-17-2017, 01:48 PM   #10
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Default Re: P0340 code

I believe Engine, ABS, CC and Transmission are the only PCM options you need to worry about as far as compatibility goes.
The real fun is getting the "new" PCM married to the "old" BCM, which looks like it's beyond the ability of the OBDLink SX (it is not a Tech-II level tool) if it doesn't marry the easy way.

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Old 11-19-2017, 08:34 PM   #11
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Default Re: P0340 code

Well, I guess I was wrong about my second ICM being good. I went to a pick-n-pull on Saturday to look for a computer but couldn't find a compatible one. But I grabbed another ICM while I was there.

And voila! It's running great now. I still think it's really weird that it worked right after I replaced the ICM the first time but then misfired again the next time I started it. I guess it's possible that it was borderline and crapped out right after I installed it (and just happened to crap out on the number 1 and 4 cylinders too).

But I drove it around a lot yesterday, and it's still running great. Thanks for the help, fetchitfido.

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Old 10-02-2019, 05:28 PM   #12
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Default Re: P0340 code

Sorry for the thread necromancy, but this problem has just reared its ugly head again. I was driving to work when it suddenly started misfiring pretty badly. My code scanner says it's misfiring on cylinders 1 and 4, and it's giving me a P0300 and P0340 code. At one point it was also giving me a P0341.

Resistance looks good on all the coil packs and plug wires. I went ahead and replaced the spark plugs even though they looked pretty decent. I swapped the coil packs with a different pair that also test within spec, and it still misfired on 1 and 4. I swapped the ICM twice (I have three—the original, one I bought from a dealer from several years ago when I was trying to track down a phantom misfire, and one I pulled from a junkyard two years ago), and it still misfired on 1 and 4.

I finally found a combination of ICM and coil packs that seemed to work, and it drove fine all the way to work and almost all the way home. Then it started misfiring again just a block from my house.

Since the coil packs all test within spec, and since the misfire stays on 1 and 4 even when I swap the coil packs from left to right, I'm pretty sure it's not the coil packs. And since all three ICMs have worked sometimes and misfired sometimes—always on 1 and 4—I'm pretty sure it's not the ICMs either.

I also backprobed circuit 633 at the PCM with the ignition on but the engine off, and it was about 4.95 volts. I haven't tested it with the engine running yet.

I tested the fuel pressure the other day, and it was fine. I haven't tested the compression, but I don't know how a compression problem would cause intermittent misfiring on just those two cylinders that would occasionally be fixed by swapping around the ignition parts.

At this point I'm wondering if it's something like an intermittent short in the wiring to the ICM. Does that sound like a possibility? Does anyone have any idea how to track down something like that? Or are there any other troubleshooting steps that I'm missing?

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Old 10-02-2019, 05:41 PM   #13
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Default Re: P0340 code

I would expect an intermittent open in the wiring more than a short. Sure, wiring/connector problems are always a possibility, that is why parts-swapping may not be successful. Intermittent can be very elusive. You might want to try installing a redundant wire from the PCM connector to the ICM connector for that CMP signal wire.

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Old 10-02-2019, 06:22 PM   #14
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Default Re: P0340 code

If it's misfiring on 1 and 4, would the open circuit just be on the wire that sends the CPS signal, or would it be on the wire that provides voltage to that coil pack? If that coil pack isn't getting the right voltage, then it would misfire on 1 and 4, and the misfire on 4 would cause the P0340/P0341, right?

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Old 10-02-2019, 08:32 PM   #15
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Default Re: P0340 code

I'm thinking it is the wire for the CMP signal, not exactly related to either #1 or 4. Yes, the #1-4 coil ciruit generates that signal in the ICM, but you have already swapped ICM and coils around. It could be the PCM, too, but that is less likely than a wiring issue and more of a hassle to try another. I would go after the wiring first. By "wiring", I am most suspicious of connectors or the wiring right at a connector.

PS: This probably isn't related to the CPS (CKP sensor) at all.

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Old 10-02-2019, 09:42 PM   #16
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Default Re: P0340 code

But it is actually misfiring, no matter what I do with the ICM and coil packs. I don't think it's just a problem with the wire that sends the CMP signal. Or is it possible for a bad CMP signal to cause a misfire on 1 and 4?

(And by the way, I meant CMP in my previous post, not CPS. I just got my abbreviations mixed up.)

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Old 10-02-2019, 09:54 PM   #17
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Default Re: P0340 code

If you actually have a misfire then replace BOTH the plugs and wires and do not use any fine wire electrode plugs either.

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Old 10-02-2019, 10:06 PM   #18
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Default Re: P0340 code

I literally just replaced the plugs on Saturday with copper Delco plugs. It ran fine for a day and then started misfiring again. The wires tested within spec too.

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Old 10-03-2019, 07:46 AM   #19
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Default Re: P0340 code

Have you replaced the wires?

Actually verify the wplug wire routing wire by wire.
Ignition wire routing as seen when looking at the engine and coils from the front, hood open, your left to right.

Engine
1-2-3-4

4-1-2-3
coil towers

Connect the matching numbers with a plug wire.

The P0340-P0341 codes are the result of ignition problems only and nothing else.

OEM ICM and coils. Plain carbon fiber wires. NGK BKR5ESA-11 plugs for DOHC or BKR4ESA-11 for SOHC. correct wire dress and routing.

Room temperature coil resistance 8k ohm minimum. The DC resistance of the #1+#4 plug wires ~12k ohm max and the DC resistance of the #2+#3 plug wires ~12k ohm max and R#1+R#4 = R#2+R#3. No super expensive wires unless you want to buy Magnecor.

The #1 and #4 plug wires must run parallel and adjacent to each other from the coil to the head.

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Old 10-08-2019, 01:28 PM   #20
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Default Re: P0340 code

I haven't replaced the wires, but they all tested within spec (though I can't remember now what the numbers were). They're all properly routed too. The coil packs all test within spec too.

On Saturday I took off the coil packs, cleaned the grounds because they looked a little rusty, and put it back together. Now it's running great again.

I really don't get it.

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