So, my inquiry is just that.

Two times within the last six week period, my truck has gone into "Reduced Engine Power", or limp mode.

The first time I chalked up to spurious, considering it was a cold start, and take off, after sitting for 24-36 hours, and figured it was my fault for not letting it warm up properly. I cranked the truck, checked my phone for messages, put it in Drive, and moved about 3 feet, and it suddenly went into "Reduced Engine Power". The second time, was last night. This one caught me by surprise. I let the truck warm up for about 20 minutes (its been consistently at or below 32F. I drove away and made it about a mile or so away, and on decelerating to a stop sign, it went into this "limp mode" again.

Parameters at the second time were:
Alternator: >14V
Oil Pressure: ~50psi
Coolant: ~210F
Fuel: >3/4 tank (less than a week old)
Speed: ~20mph
Trans: >100F, barely.

I am at a loss for cause. Everything is in good working order. Fresh oil change, sufficient coolant level in the expansion tank, transmission fluid is old, but sufficient level, and minimal or no deposits on the dipstick.

The truck has about 140KMi on it and I've had it since May 2014. I drove it from TX to CT about 6 days after buying it, with no issues to note. I love this truck, but I cannot have it doing things like this, and I have no reference to start with. Any and all help will be very much appreciated.

Thanks!
Mike

Would really help if you could pull any dtcs that are set.

Every time I take it to get the code checked (thank you for reminding me), it is on the 3rd start-up sequence. By then, the code has cleared itself. Also meaning, the check engine light has cleared.

What would really help is get the code pulled as soon as the message pops on and not shutting the truck off. The Pcm will keep the code stored until a certain amount of ignition cycles are met.

You also mentioned its been below 32 degrees. If your truck is equipped with a drive by wire system( No throttle cable at the throttle body). Ive seen enough throttle body sensors go bad in these cold spells, cause a reduced power concern.

I finally got the code from it. "P1516" - "Commanded vs Actual TP Performance (TAC Module)"

I'm thinking either the Throttle Position Sensor, or associated cabling.

I am unfamiliar with what a TAC Module is. I know it is a component in the FBW Throttle System.

Can anyone confirm or point me in the right direction?

Thanks!
Mike

The 4 main components are the pedal, associated wiring harness,the TAC controller, located probably around the brake booster and the TB.
Is the throttle body clean and operating smoothly?
This is where a factory service manual would really help, and hope someone may have the electrical tests for this system.
Start by inspecting the wiring and harness going into the TB and trace it backwards looking for cuts or damage. Ensure connectors are secure & not broken, Check connector wires going into TAC.
Also I would call a GM dealer and ask if there are any bulletins on this system for your truck. (P1516) when cold.

I gave all of the cables a good once over, and checked all the connections tight. Everything seemed OK to me.

The other night, it happened again, this time with my cruise set.

So, finally, I got fed up and took it to a local shop. He hooked up what looked like to me a pretty fancy computer scanner, and pulled up a whole lot of data, that may have led to a diagnosis. There were two history codes, and two failure reports. They looked something like this:

(1)
Temp at failure: 201F
Engine speed at failure: 17XX RPM
Time running: 48:XX (MM:SS)
APP Desired position: 31%
TP Desired position: 18%
TP Indicated position: 26%
Road Speed: 35 MPH


(2)
Temp at failure: 201F
Engine speed at failure: 18XX RPM
Time running: 21:XX (MM:SS)
APP Desired position: 0%
TP Desired position: 26%
TP Indicated position: 26%
Speed: 44 MPH

For the second event, to explain the 0% APP Desired position, I had my cruise set going down the road. It still gave a failure code, with cruise control engaged. I believe this has led myself, and the shop to believe that the failure point is the TPS mounted (riveted) to the throttle body. Due to its design, it may require the entire throttle body to be replaced. I know there are aftermarket ones that require the rivets to be drilled out, and the TPS bolted in, but is there any issue with calibration of the unit? The difference in cost between the two components, is about $100.

Anyone have any other thoughts on the matter?

Thanks!
Mike

Lets get you to double check that connector by doing the tug test.
Found this video.
Also confirm the TB is clean, spray it out with TB cleaner and then check for smooth operation while the vehicle is running.
The TB butterfly should open & close smoothly, without any hesitations in movement.

Throttle body is good. Basically the shop did a diagnostic on the Throttle body, and ordered a new one and plugged it in via the TPS, but not mechanically. What they found was the computer read the same parameters as the old one. The parameters would read 26% open at idle, then when you depress the throttle, it would close more before it opened. Even a brand new throttle body is mechanically open and has room to shut more straight out of the box. It has a straight through port for the idle (no IAC) so the slack can't be for that.

Any ideas?

Also too on a potentially related, but maybe not related note, my instrument cluster doesn't always come on after ignition. To be honest, it doesn't register anything until the vehicle is running, and what I'm saying is sometimes not even then. Now it'* come to a point where I do not think my dash isn't coming back. I do know for a fact that the shop could not actually see any computed data on their scanner until the dash lit up, when the truck was running.

Any thoughts on either issue? Could they be related even though the truck doesn't go into "reduced engine power" and have no instruments at the same time?

Thanks!
-Mike
Thanks