11 Yukon XL Denali Electrical Issue P0621 SOLVED

[SteelBaller3]

Hello, new to the group but long time stalker.
I bought this Yukon a few months ago and after a month had a shop AFM delete it.
Now I’m having some kind of electrical issue. I have a ton of pending codes but one consistent P0621. Most of the codes pending seem electrical. Is it possible my ECM is bad? How would I test that? I have no problems with it starting or any lights on dash.
When I turn the key on without starting the voltage drops quickly and battery light turns on. That’s when P0621 appears.
So far I’ve changed current sensor and negative battery ground and ground strap. Alternator is 1 year old and battery testing good. It seems to misfire and does on a couple cylinders but count is low. I changed plugs and wires but still does.
Any advice is greatly appreciated.

 

[Fless]

Here's the diagnostic path for that code, but when you get there verify that I selected the correct vehicle, or navigate to yours.

P0621 — 2011 GMC Yukon XL Denali 2WD V8-6.2L Service Manual | Operation CHARM

Detailed repair manual for the 2011 GMC Yukon XL Denali 2WD V8-6.2L.

charm.li

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

These trucks are supersensitive to low voltage in their electrical systems, and it creates havoc, not to mention setting off multiple trouble codes. So start with the basics.

"battery testing good" - How did you test the battery? The only true way to test a battery is to remove it from the truck, take it to an auto parts store with a battery bench load tester, and get it load tested.

"Alternator is 1 year old" - is it a GM OEM alternator, or is it an aftermarket one?

"changed current sensor and negative battery ground and ground strap" - How about the positive battery cable and the cable to the alternator? Are they in good condition with clean and tight connections? The high load power wires tend to rot from the inside out, so test them with an Ohm meter.

Did you check the major ground connection on the frame approximately below the driver's feet? If that ground connection is not clean and tight, you will have electrical issues.

Other members of this Forum will chime in with additional suggestions.

 

[Fless]

I'd also be interested in the other codes that are being experienced.

 

[SteelBaller3]

Yes

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

These trucks are supersensitive to low voltage in their electrical systems, and it creates havoc, not to mention setting off multiple trouble codes. So start with the basics.

"battery testing good" - How did you test the battery? The only true way to test a battery is to remove it from the truck, take it to an auto parts store with a battery bench load tester, and get it load tested.

"Alternator is 1 year old" - is it a GM OEM alternator, or is it an aftermarket one?

"changed current sensor and negative battery ground and ground strap" - How about the positive battery cable and the cable to the alternator? Are they in good condition with clean and tight connections? The high load power wires tend to rot from the inside out, so test them with an Ohm meter.

Did you check the major ground connection on the frame approximately below the driver's feet? If that ground connection is not clean and tight, you will have electrical issues.

Other members of this Forum will chime in with additional suggestions.

Yes, thank you very much. I replace with all OEM parts. Alternator is AC Delco, along with all the other parts I mentioned. I am getting positive terminals in the mail from rockauto Wednesday. I’m going to try and run more tests. All terminals have been cleaned and seem fine but I’m not as good with electrical testing. I’m using a multimeter to verify the battery and cables. But I’m going to do more tests when I finish working Wednesday. I’m not sure if the alternator was replaced with remanufacrured ac delco or new. I just have the receipt the previous owner gave me.

 

[SteelBaller3]

I'd also be interested in the other codes that are being experienced.

I checked voltage at battery, alternator, and starter last night. All 12.55

 

[mikez71]

no more other codes you mentioned?

what happens if you unplug connector at alternator?

 

[SteelBaller3]

no more other codes you mentioned?

what happens if you unplug connector at alternator?

Yes the other codes are for voltage issues at a bunch of sensors. So I’m starting to think the alternators internal regulator is overcharging? To test that I would need to charge it and keep multimeter on battery to see voltage spikes or drops?

 

[SteelBaller3]

Yes the other codes are for voltage issues at a bunch of sensors. So I’m starting to think the alternators internal regulator is overcharging? To test that I would need to charge it and keep multimeter on battery to see voltage spikes or drops?

*** start not charge

Here's the diagnostic path for that code, but when you get there verify that I selected the correct vehicle, or navigate to yours.

P0621 — 2011 GMC Yukon XL Denali 2WD V8-6.2L Service Manual | Operation CHARM

Detailed repair manual for the 2011 GMC Yukon XL Denali 2WD V8-6.2L.

charm.li

im doing this diag when I finish work Wednesday

 

[mikez71]

You said the voltage drops fast with key on, no start?

You could try disconnecting the alternator, and see if the voltage still drops.
Don't let the alt cable ground out when you disconnect it..

My guess is a short in the alternator maybe?
Computer modules shouldn't draw enough to make your voltage drop fast I'm thinking..

 

[Fless]

What are the results of a proper load test on the battery, as @Joseph Garcia recommended?

 

[SteelBaller3]

Sorry for the late replies, I have been busy at work. I work 12 hr shifts so it’s hard to do any diag work after work but I finish up today and will do more testing.
I’m going to take the battery and alternator to autozone to have them test tomorrow.
Also going to go through my relays and fuses to make sure they’re all good.
That’s a great idea mikesz71 I will try that as well.
It runs really rough and I believe it’s from this electrical issue because it didn’t before this.
I will report my findings soon hopefully.

 

[SteelBaller3]

Tested fuses all good. Took power off alternator with key on, still did it. Going to take battery and alternator to get tested. Also started it and with idle going up and down I did a graph of acc sensors and tps. Seems off to me but I’ve never done this kind of testing on a vehicle with so many sensors so any insight is appreciated. This was at idle

 

[Fless]

15.25v is pretty high for a charging voltage. It'll be interesting to see if the alternator is out of spec.

 

[mikez71]

I haven't logged pedal position, but that looks off. Guessing you are not pushing pedal?

What's the voltage when running? Is it jumping around too? (log with scanner)

@Joseph Garcia maybe onto something with the grounds..Even though your battery connections are good, your electronics/wiring ground connections maybe bad.

Did you check the major ground connection on the frame approximately below the driver's feet? If that ground connection is not clean and tight, you will have electrical issues.

 

[SteelBaller3]

I haven't logged pedal position, but that looks off. Guessing you are not pushing pedal?

What's the voltage when running? Is it jumping around too?

@Joseph Garcia maybe onto something with the grounds..Even though your battery connections are good, your electronics/wiring ground connections maybe bad.

Only grounds I’ve changed are neg terminals at battery to front of frame and front of engine. Also ground strap rear driver side. I will check those grounds. Also yes this is idle no pressure on accelerator. And also, battery voltage showed around 15.1 doesn’t jump too much around .25 at when I checked while running.

 

[SteelBaller3]

15.25v is pretty high for a charging voltage. It'll be interesting to see if the alternator is out of spec.

Yeah I’ve seen it as high as 15.5

 

[mikez71]

Voltage not necessarily too high, the charging system allows that when it thinks it's needed.
If your modules show low voltage due to a bad ground, it would be cranking up the alternator voltage.

Since you have a scanner, try and look for 'control module voltage' or something like that.

 

[SteelBaller3]

Voltage not necessarily too high, the charging system allows that when it thinks it's needed.
If your modules show low voltage due to a bad ground, it would be cranking up the alternator voltage.

Since you have a scanner, try and look for 'control module voltage' or something like that.

 

[mikez71]

Graph like you did with pedal position might show better if its fluctuating, with engine running.