NEED HELP Multiple codes and reduced engine lite on

scott414

Member
have a 05 trailblazer with mult codes. Recent plug and coil change ( ac delco 41-103) as was running rough and reduced lite on before plug change. Now with P0068 that occurs with reduced engine lite also, and p0304, and p0300 at times. I can reset, and will be looking at cylinder 4 again. I did save freeze frame info if needed. also did clean TB and relearned.
 

TequilaWarrior

Well-Known Member
While running, disconnect 1 coil at a time. If disconnecting a particular coil does not make it worse - that coil is not firing. Then the trick is to find out WHY. Even new coils and plugs fail sometimes. You already have a code pointing to cylinder 4. If you disconnect the coil on cylinder 4 and there is no change, that's pretty much confirmation that you have a misfire there. If you kept your old coils, swap out bad ones found in the disconnect test for known good ones that you kept. You can also rule out or confirm a bad coil by finding the misfiring cylinder (and confirming by plugging, unplugging at least a couple times while running). Then swapping that coil with another you can confirm is good. If the misfire follows the coil - it's the coil. If the misfire does not follow the coil, it's ~probably~ the plug.
 

MRRSM

Lifetime VIP Supporter
Welcome to GMT Nation...

To address your P0068 Trouble Code... The Most Likely Culprit due to its almost Constant Use is your Accelerator Pedal. Unlike the Old Days where the "Gas Pedal" was an Analog System of having a Wound Wire Cable and Bell Crank assembly that Physically Opened the Throttle Bodies of Carburetors that had Coiled Springs to Close them at Idle... The Modern ACC Pedal is just a "Package of Electrical Potentiometers" that sends "positional" and "rate of change" information to the PCM and to the Electronic-Motor Driven Throttle Body to adjust the "Butter-Fly Plate". The GM ACC Pedal uses two distinct High and Low Voltage Signals along with a Class 2 Databus 5 Volt Reference Signal to and from the PCM to achieve its functions.

If it Fails... The PCM is Coded to Automatically Drop into a Reduced Power Mode... especially if the Vehicle is engaged to Drive in Reverse. This Safety Consideration is called "Going Into Limp Mode" and thereby points to either the ACC Pedal ...or the Throttle Body as being the possible Faulty Components. There are Videos on Youtube made to show you "How to Diagnose the ACC Pedal using a Digital Multi-Meter". I'm suggesting that you Start by Looking for an ACC Pedal problem First...as getting a replacement is only around $40.00 and that is much less Pricey than just purchasing a New GM 4.2L Throttle Body without knowing if that component is at fault.

The P0300 Codes can have causes ranging from a Dirty MAF/IAT to a Failed Upstream O2 Sensor to having Loose Intake Manifold Bolts and/or other Vacuum Hoses on the Engine leaking. Study this Video for the means to improve your understanding of "The Modern Drive-By-Wire System" that is at the Heart of this issue:

 
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scott414

Member
I want to thank both of you gentlemen for your excellent suggestions, I will be looking at my trailblazer as soon as I get home.
 
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scott414

Member
Welcome to GMT Nation...

To address your P0068 Trouble Code... The Most Likely Culprit due to its almost Constant Use is your Accelerator Pedal. Unlike the Old Days where the "Gas Pedal" was an Analog System of having a Wound Wire Cable and Bell Crank assembly that Physically Opened the Throttle Bodies of Carburetors that had Coiled Springs to Close them at Idle... The Modern ACC Pedal is just a "Package of Electrical Potentiometers" that sends "positional" and "rate of change" information to the PCM and to the Electronic-Motor Driven Throttle Body to adjust the "Butter-Fly Plate". The GM ACC Pedal uses two distinct High and Low Voltage Signals along with a Class 2 Databus 5 Volt Reference Signal to and from the PCM to achieve its functions.

If it Fails... The PCM is Coded to Automatically Drop into a Reduced Power Mode... especially if the Vehicle is engaged to Drive in Reverse. This Safety Consideration is called "Going Into Limp Mode" and thereby points to either the ACC Pedal ...or the Throttle Body as being the possible Faulty Components. There are Videos on Youtube made to show you "How to Diagnose the ACC Pedal using a Digital Multi-Meter". I'm suggesting that you Start by Looking for an ACC Pedal problem First...as getting a replacement is only around $40.00 and that is much less Pricey than just purchasing a New GM 4.2L Throttle Body without knowing if that component is at fault.

The P0300 Codes can have causes ranging from a Dirty MAF/IAT to a Failed Upstream O2 Sensor to having Loose Intake Manifold Bolts and/or other Vacuum Hoses on the Engine leaking. Study this Video for the means to improve your understanding of "The Modern Drive-By-Wire System" that is at the Heart of this issue:

One question I have regarding the gas pedal when I got that p0068 code and the reduced engine power I wasn't driving my truck it was sitting in the driveway I was letting it idle to clear the computer codes is this ain't in any other concerned that you can think of?
 

MRRSM

Lifetime VIP Supporter
***Please see my next response for a Correction about this Information Thanks to...and Courtesy of @Mooseman:

If the MAF/IAT (Mass Air Flow /Inlet Air Temperature) Sensor had a sudden Electrical Failure from a Bad Connector, Loose Connector Pin(s) or Hidden Broken or Stretched Wires inside of the Wiring Harness.... Any of these conditions could cause the PCM to suddenly Drop into a "Limp Home Mode" as well to protect the Engine from Damage at Higher RPM....even when at Engine Idle.

THIS portion of the Information is still valid:

You can investigate (and Clean the MAF Interior) using ONLY CRC MAF Electronic Sensor Spray Solvent and remove and reconnect the Harness Wiring Connector after examining it for Loose or Disconnected Wires as well. THIS is the only stuff to use (Do NOT Physically Touch the Vertical MAF Sensor Inside). And while you're at it... Replace your Air Filter at the same instance. Trust me... If the Interior of the MAF Sensor is Dirty... that Filter probably already resembles a "Rectangular, Corrugated Paper Toilet":


CRCMAFSPRAY.jpg
 
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Mooseman

Moderator
MAF would not cause REP or P0068 . I have run mine with it disconnected and it still drove normally (albeit with a code for the MAF). Not a bad idea to clean the MAF but it's unrelated to the current problems.

For the P0068 and REP, something between the pedal sensor and throttle body is not jiving. Either the pedal sensor or the throttle body is defective. Could also be something with wiring. Get the schematics from the manuals (link in my signature) and check for proper continuity between the pedal, throttle body and PCM. If you have a pick-n-pull yard nearby, you could grab a pedal and throttle body and swap them out one at a time.

If you disconnected the battery at anytime, you need to clean the throttle body. It should be cleaned out regardless and the battery disconnected for 30 minutes minimum.

For the misfires, with your freeze frame data, swap the coils between 4 and the one with the least number of misfires. If the misfire follows the coil, then it's bad. If it stays at 4, then something else is going on.

Usually it throws a P0300 until it decides which cylinder is misfiring after a while.

Check the above and report back.
 

MRRSM

Lifetime VIP Supporter
I seldom do this… But ...as I strive very hard Never to provide Advice that I cannot back with up with verifiable facts and proper research it is with that in mind that in this particular instance, I’ll have to very respectfully disagree with @Mooseman that the MAF/IAT Sensor(s) and Sensor Harness integrity plays NO PART in the Diagnosis of this particular “Limp Mode” Problem.

I can understand that ordinarily, the focus of attention would be on the ACC Pedal vs. the Throttle Body Sensor vs. the PCM, and since General Motors seldom “invents” Isolated OBD2 Technologies that will not translate equally across ALL Breeds of Engines, Transmissions and Platforms, the Information I gleaned about this MAF/IAT/HARNESS Problem some time ago from this Video holds to be True and Valid in THIS Instance; regardless of the LACK of any MAF Codes setting and especially if the MAF issue has to do with SHORTED INTERNAL HARNESS WIRING.

Please Follow On from right at 16:21 into this “GM Limp Mode Diagnostic and Solution” Video to understand WHY I answered the OP’s Follow Up Question with the ‘fringe idea’ that the MAF Sensor Circuit and Wiring Harness should NOT be overlooked during his Diagnostic Cascade.

Please Pay Particular Attention to what happens after 30:40 into this Video (and again at 33:50)...as the reality is that the MAF and IAT are on the SAME Connector. The “Door Bell” Warning Sound inside the Cab Goes Off as soon as the VOP touches the TAN Wire of the MAF/IAT Connector with his Probe ...and upon Checking the Dashboard Panel for Messages...he realizes that whatever he touched ...triggered in the Engine Bay Caused the Vehicle to Enter into “Limp Mode”. The MAF and IAT are Integral...so a failure in either Harness Circuit Integrity could make this problem happen… and apparently, it DID:



But... I also have to be able to Admit it whenever I Make Mistakes.... and in this instance... BOTH @Mooseman and I are VINDICATED. The Follow On Video shows that the VOP engaged in some Unintended Misdirection that happened when the VOP Started out working on the IAT Circuit; but then he Cuts his Video immediately back to the Work he was performing the Loose Connection-Broken Wire of the TPS 1 and TPS 2 Circuits.

So what he ACTUALLY did was...Accidentally Bump the TPS Harness while working on the IAT Connector Probing. He later discovers that the IAT TAN Wire was ALSO Damaged along with the TPS1 Circuit Wires inside of the Wire Harness Bundle, So these are Two Separate and Distinct Issues... that are Connected by Wire Cut Damage...and so @Mooseman is CORRECT and I stand as being Corrected . I much prefer this as an Outcome. :>)
 
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scott414

Member
No Maf sensor, have map sensor which did throw a
P0106 code yesterday. Changed #4 spark plug as indicated and seems to be running on now. Also new map sensor too. Fingers crossed
 

MRRSM

Lifetime VIP Supporter
On occasion... The MAP Sensor can become congested with Engine Oil Inhaled from the Air Intake Plenum (Positive Crankcase Ventilation) Hose and needs to either Cleaned Out or replaced. (Toes Crossed... And Good Luck...) :>)
 
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scott414

Member
Well this seems to be an intermittent issue, no rhyme or reason. Checked and all looking good, but lucky I had my code reader with me and able reset while away home . YUP P0068
 

MRRSM

Lifetime VIP Supporter
This may sound a bit weird; .given that we KNOW that the problem is either with the Throttle Body or with the Accelerator Pedal... But FWIW... I'll ask you this:

Do you have an Aftermarket CAI (Cold Air Induction) Set Up on your Vehicle? If so, take it apart to point where you can examine and see if the Air Filter is Over-Oiled and Contaminated... and remove it.. Then... Try Clearing the P0068 Code and taking it for a VERY Short Test Drive. If this solves the Problem... You should consider returning to using the "Chockablock----Stock-As-A-Clock" Air Box and ... A Brand New Filter.

Even if you Do NOT have a CAI.... Try removing the Stock Filter, Clear the P0068 Code and Perform the Test Drive in the same manner. Let us know if this Odd Idea works to Solve this issue. Otherwise... You still only have the other Two Items to consider Testing and/or Replacing.
 
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scott414

Member
This may sound a bit weird; .given that we KNOW that the problem is either with the Throttle Body or with the Accelerator Pedal... But FWIW... I'll ask you this:

Do you have an Aftermarket CAI (Cold Air Induction) Set Up on your Vehicle? If so, take it apart to point where you can examine and see if the Air Filter is Over-Oiled and Contaminated... and remove it.. Then... Try Clearing the P0068 Code and taking it for a VERY Short Test Drive. If this solves the Problem... You should consider returning to using the "Chockablock----Stock-As-A-Clock" Air Box and ... A Brand New Filter.

Even if you Do NOT have a CAI.... Try removing the Stock Filter, Clear the P0068 Code and Perform the Test Drive in the same manner. Let us know if this Odd Idea works to Solve this issue. Otherwise... You still only have the other Two Items to consider Testing and/or Replacing.
Mrrsm- everything in my 05 is stock, no oil on the air filter, resonator is clean. I did drive like mentioned, no CEC lites on, scanned and no codes. Has been like this all day yesterday. These intermittent problems are a pain to solve
 

MRRSM

Lifetime VIP Supporter
From Eric The Car Guy... after HE got some Help from "ScannerDanner":

https://www.ericthecarguy.com/kunena/8-Service-and-Repair-Questions-Answered-Here/17989-p0068

MAPTPSPROBLEM.jpg

Here is a Complete GM Trouble Codes Definition PDF so you can Cross Check ScannerDanner's Observation. The "Key" Diagnostic Observation you've made so far... is that this Problem is "intermittent" so it may well be that the Throttle Body Harness has a Loose or Broken Wire...or as has happened in the past... a Loose Pin in the TPS Harness Connector.


Amazon sells quite a few Replacement Connectors with Pigtails that can be R&Rd for the Old One if it turns out to be the problem. Diagnosing the TB/Envoy Throttle Body as a completely self-contained unit is well nigh impossible because the Genius who designed the Damned Thing tried to make it as "Solid State" and inaccessible as possible.


 
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scott414

Member
so.... on way to work, p0068 x 3 times, was able to reset each time. 2 while driving; variable city traffic, once while at stop sign. Im leaning right now towards throttle body as this correlates with the code, wouldnt the pedal sensor throw a different code if it was bad??? Frustrated beyond belief!!!
 

MRRSM

Lifetime VIP Supporter
You should consider performing an R&R and examination of the TPS Connector and ALL of the Pins and Related Wires in the adjacent Harness, Also... try removing the Four Bolts holding the Throttle Body to the Intake Manifold to perform a careful, light action "Shake Test" of it while listening for anything that might be loose or 'rattling around' underneath the Drive Motor Cover. If you have not as yet tried to do these things, then I'm not sure that you have reached the point of having to break down and just Buy a New Throttle Body.

However ...if it comes down to cases ...and this TB Swap does become necessary.... the one you need is available on Amazon... and you should also check on RockAuto.com to see if they carry this item, too. While you're looking around on RA... Get a Brand New, Replacement "Intake Manifold" Throttle Body "O" Ring Style Gasket as well:




2005TBTHROTTLEBODY1.jpg




PS... FYI... The Engine Data in your Profile presently lists it as being a "4.3L" and "V6."... (Easy enough to edit...)
 
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Mooseman

Moderator
Before spending $200 on a new throttle body, I would just source a used one, hopefully from a pick-n-pull, for a lot cheaper. These are not known to fail a lot but they do occasionally. If you pull one, give it a shake test and listen for anything loose. If it's silent, it's likely good. And pick-n-pulls usually have good return policies.
 

gmcman

Well-Known Member
I had the wonderful misfortune of someone...somehow, accessing my engine bay while at work a number of years ago. They cut my TB harness, maybe in hopes I would leave it.

I didn't realize at the time it was cut, the dealer stays open till 11 PM and was 2 miles away, was in REP mode. I actually managed to get about 1000 RPM when pressing the gas pedal, one or 2 wires were still touching.

They fixed the harness but could not get the Tech 2 to read the TB and its correct function.

Took them about 3 or 4 tries and finally came down to a bad crimp. So make sure if you have removed your TB harness at anytime, triple check that none of the wires have pulled out.

Like what has been stated...ANY discrepancy between the pedal and TB will put it in REP mode.
 

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