Originally Posted by cobalt09

Hello, I recently experienced a similar issue on my 2009 Cobalt LT after 62,000mi and managed to root cause the issue to a wiring harness problem that affects many GM/Pontiac models.


In short, there is a wiring harness that contains a number of wires that has two really bad chafe points that rub on the oil tank filter under the hood. Over time the harness insulation rubs against the sharp edge of the oil tank filter and eventually can make contact with some of the copper wires. The fix is to wire-tie the wire harness so that it does not rub against the oil tank filter (<$1.00) and a bit of electrical tape around the wire that had insulation cut.

In my particular case, my OBD2 reader was throwing P0641 "Sensor Reference Voltage A Circuit/Open" trouble code. In the wiring schematic, this is known as the "5 Volt Reference (1)" which connects to the fuel tank pressure (FTP) sensor and the accelerator position pedal sensor (APP). Initially, I didn't have time to sit down and play with this so I took it to the shop and my mechanic replaced the entire pedal and APP sensor ($150!) but the issue quickly returned. I would be driving and everything would be find but then all of a sudden I would feel a jerk and the car would lose the ability to accelerate and "ENG PWR REDUCED" and "SERVICE TRACTION" would appear on the display.

The annoying part of this whole thing is that the issue is intermittent and depends entirely upon whether the metal oil tank edge makes contact with the wires. The car was not reliable and my mechanics weren't interested in spending time debugging it. I ended up checking all of the sensors connected to the 5V supply and they were all functioning fine and read 4.8~5V at the sensor.

My breakthrough was when I was fortunate enough to stumble upon the following video below which solved the issue (this fix should honestly be stickied for all GM owners). His symptom was a similar trouble code P0642 "Sensor Reference Voltage B Circuit/Open" but it's the same root issue and the code you will see will depend on which wire in the harness is damaged.
https://www.youtube.com/watch?v=1uQbvc7qEvY

Since the wire-tie fix, the trouble code has completely cleared and I have not had any "ENG PWR REDUCED" or "SERVICE TRACTION" issues for over a month (driving each day).

If the solution works out, definitely help the YouTuber out with likes/subscriptions. I unfortunately didn't see the video before an unnecessary pedal replacement but saved me from selling the car for scraps.

Hope this helps...

JP

Originally Posted by hanky

Thank you for sharing that info.
Unfortunately, most folks have no idea what a 5 volt reference is and where it is used.
Hopefully, anyone with the problem will share your fix, thanks again.

Originally Posted by acamp88

Hey everyone, I am new here & was hoping for some help with mine & my boyfriends 2010 Cobalt LT. Just over 35,000 miles. Purchased as a Certified Preowned vehicle from an Authorized Chevy Dealership.

This morning I left for an extremely important meeting & got in the car and turned it on. Was fiddling with my iPod for a few minutes and when I go to take off notice the check engine light is on. Think hmm, maybe I didn't turn the car on all the way. Remove key, start car again. This time "Engine Power Reduced" message and car makes a sound as though it's powering down, yet still running. Power steering is not working/very stiff. I turn the car off again & think GD these recalls! Just got another for the power steering and figure this was the problem. Start the car again and THIS time it seems like all the service lights are blinking (though everything happened so quickly could have just been them turning on and off like when you start the car) the needles on the gauges are flickering and the car starts shaking violently all while displaying the check engine light, the traction light, the info light, and the screen reading "engine power reduced". I turned the car off immediately as it was terrifying and I thought I was going to explode.

Freaked out in the car for 20 minutes as I was now going to miss my appointment/couldn't contact the people. Started the car again to take a video but this time check engine light is still on, no reduced power, and running smoothly. Oh good. Called to make an appointment with the dealer for an hour later thinking I should be safe to at least make it there- 4 miles. WRONG.

I get a few blocks away to a very major four lane road where people often go way above the speed limit. The car suddenly slows to about 5 mph while going 40, lunging forward, the traction lights are blinking, check engine light still on, and screen switching back & forth between "service traction" & "engine power reduced!" The car was LUNGING INTO TRAFFIC. Even with my foot slamming on the break. It was still lunging. Scariest moment of my life. The car was basically coasting at about 5 mph while cars are are beeping at me & I'm almost rear ending the car in front of me at the red light as it wouldn't stop lunging. Thank god I wasn't far from home. Immediately turned onto a side street, coasted home while lunging the whole way and the car started shaking violently again. I literally thought I was going to explode.


SO, now I have got to have it towed by my Geico Roadside Assistance, but problem is- once it gets wherever they tow it, if I don't choose to get it fixed there I'm screwed & have to pay to have it towed elsewhere. Dealership wants $140 just to look at it..... I've seen a few threads about this problem in Malibus. We are so frustrated and have had nothing but problems with this car that we bought to replace a 2009 Cobalt that got totaled. Can't afford to dump thousands into this thing for whatever mechanic of the day thinks may be the problem only for it to return later. The mechanics at my dealership are so freaking rude too!

Ok, rant over. Any insight from all the Chevy savy/car brainiacs out there?