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Bad PCM symptoms??

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Old 01-14-2010, 10:08 AM   #1
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Default Bad PCM symptoms??

I'm suspecting my PCM may be bad...my car's running like crap...as in misfiring on like 4 cylinders but not throwing any misfire codes...we checked the coils...changed plugs and wires...put in a different maf...haven't done O2's yet

I was just wondering what kind of things happen if a PCM goes bad? Any of this sound familiar?
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Old 01-14-2010, 10:50 PM   #2
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That is not consistent with any type of PCM failure that I have seen before. Hard misfires with no codes can come from intermittent shorts to ground that do not blow fuses. Also, one misfiring cylinder will 'look' lean to your O2 sensor and drive that bank of the engine into extreme over fueling trying to correct what it sees as being lean. If it is run like this, plug fouling and additional misfires from over fueling can follow.
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Old 01-15-2010, 08:11 AM   #3
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Bad LS based PCMs are not a common occurrence. Misdiagnosis of them is far more common.

I would look elsewhere for the cause of the misfires. Just because the computer does not recognize them does not mean it's bad.
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Old 01-15-2010, 11:19 AM   #4
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Quote:
Originally Posted by Frost View Post
That is not consistent with any type of PCM failure that I have seen before. Hard misfires with no codes can come from intermittent shorts to ground that do not blow fuses. Also, one misfiring cylinder will 'look' lean to your O2 sensor and drive that bank of the engine into extreme over fueling trying to correct what it sees as being lean. If it is run like this, plug fouling and additional misfires from over fueling can follow.
So where should I look for these grounds that would cause these misfires? And would bad front o2's cause this to be even worse?
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Old 01-16-2010, 02:22 PM   #5
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bump...taking it to the mechanic monday if I can't get this figured out
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Old 01-22-2010, 09:44 PM   #6
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Wasn't a pcm issue...I had 1! valvespring going bad that eventually broke causing all my problems...changed the spring and now the thing runs like a top
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Old 02-15-2012, 08:30 AM   #7
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Quote:
Originally Posted by Frost View Post
That is not consistent with any type of PCM failure that I have seen before.
I know this is an old thread, but what are the typical symptoms of PCM failure?

One day I drove my T/A home (stock PCM), waited 30 minutes and then it wouldn't start (crank, no start) when I went to leave. I went around making sure the grounds were tight, checking fuel pressure, and then one time after not changing anything, it starts. Started fine the next dozen times, moving it around in the driveway. A few days later, confidence rebuilt, it left me stranded on the side of the road a few miles later. It just died and cranked but wouldn't start again. Trailered it home and sure enough, fire up right away getting it off the trailer and ran fine. There goes any diagnosis.

Last night, it starts fine and I was poking around under the hood. "Feels" like an electrical fuel issue. I gave the PCM and surrounding wires a wiggle and it died, wouldn't restart. YES! Moved the PCM/wires, starts. Unwrapped the wired from the loom to the PCM and didn't see any chaffing or exposed wire. More wiggling and I couldn't get it to die again. Got dark, gave up.

I'm 90% it's going to be a broken wire or chaffed wire I've not yet discovered. HOWEVER, there are some other slightly odd things that happen and want to rule out the PCM. For one, after a minute or two sitting at a stoplight or whatever, the fuel gauage will drop to zero for a few moments, sometimes until I start moving. And then, when it started on the trailer, I noticed the tach read zero RPM's for five seconds or so when it first started.
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Old 05-08-2014, 09:17 PM   #8
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Bump, alien did you solve this problem? I'm trying to rule out a pcm failure in my cts-v.

Last edited by Lbar; 05-09-2014 at 12:10 AM.
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Old 05-10-2014, 09:35 AM   #9
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The only non-physically damaged bad GM PCM I've experienced was in my 02 WS6. I had an intermittent cylinder 5 misfire that kept getting worse. All the usual troubleshooting including re-flashing the PCM didn't resolve my problem. As a last variable I threw a spare PCM in the car and the problem cleared. I moved my suspected bad PCM to another car and the problem followed even after flashing for that combination.

The true test for a bad PCM would be swap with a known good one (if you have that ability) and see if the problem clears. Even better if you have access to two vehicles with the same style PCM and swap between them like I did and see if the problem follows.
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Old 05-11-2014, 01:40 AM   #10
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Thank you for replying gofast. I have 05 CTS-V and I don't know anyone who has one. The PCM is right under the splash guard, behind the driver side fog light. A quick swap if I can find someone willing. I will ask around.

Here is my issue(s) so far. It began with bad camshaft position sensor codes. A P0371, Timing Reference High Resolution Signal A Too Many Pulses and a P0372 Timing Reference High Resolution Signal A Too Few Pulses code. I had a occasional hard start with backfiring into the intake and a little later a random bucking after warmed up with a certain amount of throttle lightly accelerating in lower gears.

I replaced the cam sensor with a new Delco part. I did this at my friends shop. The first start up with the new sensor was a hard start just like before. My tuner friend said I did not need a crank relearn. On the first start up I had a new problem, a P0118 Engine Coolant Temperature Circuit High Input and one of the cam sensor codes returned, P0342. He hooked his HP Tuners up and when he cleared the codes my temp sensor would come back on for 15 seconds before throwing both codes again and dropping my temp gauge to cold and kicking my radiator fans on. He said that one of my o2 sensors was showing rich without being out of spec. However, the occasional bucking seems to have stopped.

His guess was a bad PCM because there was too much wonky stuff going on. But since he only has experience with 1 bad PCM in his career and the symptoms seemed much different to me I'm not going to run out and get one until I've exhausted all other possibilities. I've gone through most of the grounds so far, they are fine. I got a new coolant sensor and plugged it in (didn't install it). It did not fix my temp sensor issue. Then it rained for the rest of the day so I'm postponed until tomorrow.

My thoughts are that this new camshaft position sensor is somehow faulty and causing problems. It is just too coincidental that I have issues with the coolant temp sensor wiring right after I installed the new cam sensor. I can't tell what wires they have in common but the two seem to be apart of the same feeder of wires. Tomorrow I plan to check the PCM ground and shake the wires attached at little bit. I'll check the other grounds underneath the car. If that yields nothing then I will put the old camshaft position sensor in and see if my temp gauge comes back on. If I regain my temp sensor then I will get another cam shaft sensor.
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Old 06-12-2015, 11:19 PM   #11
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Did you get this figured out?
I just purchased a 05 CTS-V with the following problems and am just getting started on trying to figure out what the root cause is. I am wondering if maybe the PCM has just lost its mind and needs replaced.
1. Cam position issue causes long starts
2. AC compressor will kick on when commanded manually via EFI Live but will not kick on automatically. Replaced pressure switch and recharged which did not fix the issue.
3. Evap Vent solenoid - this is probably just a bad valve (guess the rear cradle has to be pulled to replace it, so that sucks)
4. Today when coming home the "Info" section of the main display didn't work. It was blank even though I tried asking it to display outside temp, trip a, avg mpg, etc. After key cycle this display worked like it should.
So I have a few strange issues and am starting to wonder if at least 1, 2, and 4 may be caused by a bad PCM and not bad other parts.


Diagnostic Trouble Codes

P0342 Camshaft Position (CMP) Sensor Circuit Low Bank 1 Sensor A EPA ($10) "Powertrain Control Module (PCM)"

P0449 Evaporative Emission (EVAP) Vent Solenoid Control Circuit EPA ($10) "Powertrain Control Module (PCM)"

P0342 Camshaft Position (CMP) Sensor Circuit Low Bank 1 Sensor A EPA Pending ($10) "Powertrain Control Module (PCM)"

P0449 Evaporative Emission (EVAP) Vent Solenoid Control Circuit EPA Pending ($10) "Powertrain Control Module (PCM)"

P0342 Camshaft Position (CMP) Sensor Circuit Low Bank 1 Sensor A MHC ($10) "Powertrain Control Module (PCM)"

P0449 Evaporative Emission (EVAP) Vent Solenoid Control Circuit MHC ($10) "Powertrain Control Module (PCM)"

B0092 Passenger Presence System {single or 1} Performance HC ($58) "Inflatable Restraint Sensing and Diagnostic Module (SDM)"

U1000 Class 2 Data Link H ($58) "Inflatable Restraint Sensing and Diagnostic Module (SDM)"

P0196 Engine Oil Temperature (EOT) Sensor Performance HC ($60) "Instrument Panel Cluster (IPC)"

B1327 Device Power {single or 1} (low current 1) Circuit Low H ($60) "Instrument Panel Cluster (IPC)"

U1016 Lost Communications With Engine Controller - Device $10 H ($60) "Instrument Panel Cluster (IPC)"

B1327 Device Power {single or 1} (low current 1) Circuit Low H ($86) "Radio Antenna Module (ARS)"

U1064 Lost Communications With Body Controller - Device $40 H ($86) "Radio Antenna Module (ARS)"

U1016 Lost Communications With Engine Controller - Device $10 H ($86) "Radio Antenna Module (ARS)"

U1305 Class 2 Data Link Low or High H ($86) "Radio Antenna Module (ARS)"

C0750 Left Front Low Tire Pressure Sensor H ($86) "Radio Antenna Module (ARS)"

C0755 Right Front Low Tire Pressure Sensor H ($86) "Radio Antenna Module (ARS)"

C0760 Left Rear Low Tire Pressure Sensor H ($86) "Radio Antenna Module (ARS)"

C0765 Right Rear Low Tire Pressure Sensor H ($86) "Radio Antenna Module (ARS)"

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Old 08-04-2015, 11:06 PM   #12
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Well, didn't get too much feedback from my first response but here goes:


Tore into it and removed the intake manifold to access the cam position sensor. Ran resistance checks on both the old sensor and new sensor. Resistance values looked same.... not a good start as looking like maybe no the sensor.
Tested the connector and had continuity on one pin without the key on. With the key on I lost continuity on that pin AND did not have a reference voltage of 12V on either of the 2 other wires like I should have.


SO does this mean that the ECM/PCM is bad? I know it could also be the harness but I checked it over and it looks ok. Guess I should check continuity from sensor connector to ECM, but starting to feel like junk ECM which maybe explains some of the other weird electrical crap with this car.


I didn't try cranking on it because I have so much stuff unhooked but was looking for confirmation that just the key on on an LS6 should provide power and ground to the cam position sensor?


Thoughts?
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Old 08-05-2015, 09:13 AM   #13
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Im having a similar issue with bank 2 sensor 1 O2. It reads flat at the ECM and voltage doesnt change in the signal at all. Ive replaced o2's swapped them around with known good o2's. I took the harness apart to look at the wire for chaffing and exposed wire core. Did continuity checks at the plug. Than went back and did continuity checks at the ecm bulk harness to see if it was signal jumping with something other than itslef and still nothing. Checked pins for corrosion or looseness and still saw no issues

Everything leads me to ECM malfunction. Since I know that bank is running rich and has fouled the plugs yet im still getting a lean voltage value from the scan tool.
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Old 08-05-2015, 09:56 AM   #14
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Yeah, both of our issues seem like real head scratchers. I am going to put mine back together with a new cam position sensor and see if that magically fixes it, but I am not holding my breath. I am pretty sure I am going to be buying a new ECM. Looks like Rock Auto has them at reasonable prices. But my ECM seemed to also be reading a strange value from the AC pressure switch and even after replacing this switch and charging the system, the ECM would not engage the compressor unless commanded manually to engage the compressor via EFI Live. Unfortunately, I don't have a Tech II because the other thing that needs checked is the EVAP temp sensor, but the GM dealer said the it was the pressure switch. Which didn't fix anything.
So I am just wondering if the ECM has gone crazy and isn't reading a few sensors correctly, Cam Position and AC Pressure switch in my case. Maybe NOX sensor in your case.... Seems about the last thing it could be in your case. In my case I need to get the car back together and see if the hard start / no cam position code is fixed or not by replacing the sensor.
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Old 08-05-2015, 10:28 AM   #15
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If you still have it apart I would check the ecm harness's pins with a multimeter and make sure the signal wire from the sensors that are acting up arent reading with something else.

Atleast that way you can rule out the harness being the issue.

yeah in my case I have a 98 ecm which isnt compatible with the 99+ ECM's without repinning the harnees and in my case thats a nightmare since my ECM is under the dash now. I have found a company that will sell me a already flashed ECM (presumably to my vin#) for $200 so i think thats the route I'm going to go as i think im out of options. I dont think there is a parameter for the o2's that can be edited to do what I am seeing except disabling them, but thats not whats happening.

Best of luck hopefully you can figure yours out
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Old 08-23-2015, 05:43 PM   #16
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Default Running like crap after car wash

So I run the car through the carwash,an almost immediately it starts running like crap,acting like it was running on 7 cylinders,it through some codes running lean 02 banks both sides bad lost my trac y ion control ,catylitic defencies,its putting fuel in the oil cuz its so lean help,04 gto
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