NEED HELP P1221 Only with A/C on

Ric_Whit

Original poster
Member
Sep 24, 2012
71
Hi everyone on this form.

I have a 2002 Chevy Trailblazer 2WD 4.2L and occasionally I’m getting CEL P1221 with Reduced Power Mode specially when I driving slowly.
When this happen I got my code reader (Innova 5610) cleared the codes and started it back up the engine don’t want to start and the lights came back on tired again to clear the codes and came back on again, clear the codes and came back on again, clear the codes and came back on again.
After 5 minutes clearing code finally it clear and the engine fire up.
I realize this has been covered numerous times and I have read several posts concerning this.
I did everything what is mentions but my problem seems, to me, a little different.
I have replace the throttle and the fan clutch with GM OEM parts.
Still I getting occasionally the CEL P1221 with Reduced Power Mode but only when the A/C is running specially when I driving slowly.
I know that when the A/C is running the temp will climb a bit but not the gauge temp.
I can see the fan clutch is turning with speed and I can feel it pulling quit warm air.
I can drove my vehicle with the A/C off for weeks or months without any problem.
Again this only happens occasionally when I have the A/C on and driving slowly.

Any suggesting on where I can look to fix this?
Thanks
 

mrrsm

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Oct 22, 2015
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Besides performing an Excellent Diagnostic Approach on his "Limped Out Envoy"... The VOP (Video Original Poster) tells a Great Visual Story using his Oscilloscope, His Techniques and AND Book Knowledge to figure this problem out.

If nothing else...this Video might give you a few more places to look. The One Advantage the "O" Scope will give to ALL Mechanics is being able to actually SEE the Fault Live and In Color to help things along:

 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
I know that when the A/C is running the temp will climb a bit but not the gauge temp.
I'm kinda confused on this one. The engine temp climbs but not the gauge? Or do you mean the A/C temp climbs? In either case, I would suspect the fan clutch. How long ago was it replaced? It is a common issue of the wiring getting damaged from the sensor on the fan to the connector. You can test it to see if it's working:
How to test the electro-viscous fan clutch

You can try disconnecting it to see if the P0121 goes away. Just be aware that the A/C will get warm and keep an eye on engine temps.

You didn't mention if you replaced the pedal sensor or is that what you mean by "throttle"?
 
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Joe_67

Member
Aug 9, 2022
56
Central Virginia
That was an awesome video and great info. That 1221 code is exactly for that kind of "disagreement" in the TPS signals, and the Innova scantool should be able to give you live data on the TPS. I would get the TPS up on the scantool and start with that complete unplug test. Then start plugging things back in one at a time. If you do find it is b/c of the fan and you did RECENTLY replace the clutch, then start by looking for a fault in the fan wiring instead.
 

mrrsm

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Oct 22, 2015
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Tampa Bay Area
Oh... and if you are Wondering "WTF?!" the Electro-Viscous Fan has got to do with all of this... Keith De Fazio from New Line Auto and "Crazy Ivan" The Man from PHAD (Pine Hollow Auto Diagnostics) shot this video of a Trailblazer suffering with these identical problems.

Towards the end of THIS Diagnostic Video, while Ivan uses his Oscilloscope to view what is happening, Keith observes and surmises that as the Fan Clutch "Guts" breakdown during PWM Communication with the PCM... it somehow creates or allows an A/C signal to be generated and THIS is what puts all that "Electrical HASH" back through the 5 Volt Reference Circuit (or on the Class 2 Network) causing all of the Damned Trouble:

 

Ric_Whit

Original poster
Member
Sep 24, 2012
71
I'm kinda confused on this one. The engine temp climbs but not the gauge? Or do you mean the A/C temp climbs? In either case, I would suspect the fan clutch. How long ago was it replaced? It is a common issue of the wiring getting damaged from the sensor on the fan to the connector. You can test it to see if it's working:
How to test the electro-viscous fan clutch

You can try disconnecting it to see if the P0121 goes away. Just be aware that the A/C will get warm and keep an eye on engine temps.

You didn't mention if you replaced the pedal sensor or is that what you mean by "throttle"?
#Mooseman, well I said engine temp because I can feel the temp in the engine bay climb a bit.
The fan clutch was replace January 28, 2022 with ACDelco 15-40133.
I did the test and the fan clutch past.
Pedal sensor I did not replace, I only replace the Throttle Body with ACDelco GM Original Equipment 25312095

As I said this happen occasionally (not every time) when I have the A/C on and driving slowly.
It have to do with temp for sure.

The more notable was yesterday when this happen, I try 4 time to clear the code and the code don't want to clear at all.
I was stuck in the middle of the road.
 

Ric_Whit

Original poster
Member
Sep 24, 2012
71
That was an awesome video and great info. That 1221 code is exactly for that kind of "disagreement" in the TPS signals, and the Innova scantool should be able to give you live data on the TPS. I would get the TPS up on the scantool and start with that complete unplug test. Then start plugging things back in one at a time. If you do find it is b/c of the fan and you did RECENTLY replace the clutch, then start by looking for a fault in the fan wiring instead.
Hey Joe_67 I will try with the scan tool, hope I can recreate the P1221 code.
 

Ric_Whit

Original poster
Member
Sep 24, 2012
71
Oh... and if you are Wondering "WTF?!" the Electro-Viscous Fan has got to do with all of this... Keith De Fazio from New Line Auto and "Crazy Ivan" The Man from PHAD (Pine Hollow Auto Diagnostics) shot this video of a Trailblazer suffering with these identical problems.

Towards the end of THIS Diagnostic Video, while Ivan uses his Oscilloscope to view what is happening, Keith observes and surmises that as the Fan Clutch "Guts" breakdown during PWM Communication with the PCM... it somehow creates or allows an A/C signal to be generated and THIS is what puts all that "Electrical HASH" back through the 5 Volt Reference Circuit (or on the Class 2 Network) causing all of the Damned Trouble:

Very interesting video, I wish to have those tools.
 

mrrsm

Lifetime VIP Donor
Supporting Donor
Member
Oct 22, 2015
7,642
Tampa Bay Area
These are EXTREMELY Affordable... The Hantek Model #1008C (8) Channel Oscilloscope Kit @ around $110.00.

The Hantek Software is FREE to Download and Use Right Away at will from www.hantek.com and here at GMT Nation, We've got the Introduction and Training for this and many other Scopes at THIS Link any time you want to look things over ...and get your feet wet:



EVERYTHING that Keith and Ivan did in that Video examining the various 5 Volt Reference Signals at the PCM Connectors AND the Class 2 Network access can also be accomplished using a Decent Scanner and THIS Kit along with your Windows Laptop Computer:


91fR3rj0MDL._SL1500_.jpgA1eduwe0PgL._SL1500_.jpg
 
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