Help With SES Light (Code P0121)
Thread Starter
On The Tree
Joined: Nov 2003
Posts: 175
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From: Pacific Beach/San Diego/US Navy,LSCM
Help With SES Light (Code P0121) I had my 99Z28/M6 Autotaped because my SES light came on.
The code was P0121.
It seems to happen when I turn on the Air conditioner at idle the RPM'S drop to 625-650 then it works it self back to 1025-1045 (Desired range)
A friend of mine said to Open the Idle adjutment screw. Your IAC running Warm at about 35-40.
Any help you can gve me would be greatly appreciated
Rx Bruce/StoreKeeper99Z28
The code was P0121.
It seems to happen when I turn on the Air conditioner at idle the RPM'S drop to 625-650 then it works it self back to 1025-1045 (Desired range)
A friend of mine said to Open the Idle adjutment screw. Your IAC running Warm at about 35-40.
Any help you can gve me would be greatly appreciated
Rx Bruce/StoreKeeper99Z28
Thread Starter
On The Tree
Joined: Nov 2003
Posts: 175
Likes: 0
From: Pacific Beach/San Diego/US Navy,LSCM
Originally Posted by StoreKeeper99Z28
I had my 99Z28/M6 Autotaped because my SES light came on.
The code was P0121.
It seems to happen when I turn on the Air conditioner at idle the RPM'S drop to 625-650 then it works it self back to 1025-1045 (Desired range)
A friend of mine said to Open the Idle adjutment screw. Your IAC running Warm at about 35-40.
Any help you can gve me would be greatly appreciated
Rx Bruce/StoreKeeper99Z28
The code was P0121.
It seems to happen when I turn on the Air conditioner at idle the RPM'S drop to 625-650 then it works it self back to 1025-1045 (Desired range)
A friend of mine said to Open the Idle adjutment screw. Your IAC running Warm at about 35-40.
Any help you can gve me would be greatly appreciated
Rx Bruce/StoreKeeper99Z28
I'll give ya this one;
The TPS & the A/C low pressure transducer(cycle switch) share the same 5 volt referance from the PCM.
The transducer intermittantly shorts out & pulls the 5V referance close to 0 volt.
The result is a strange running problem & a P0121 TPS low volt.
We started seein these 6-7 years ago & the first one was a b!atch to figure out.
Now it's trivial.
The TPS & the A/C low pressure transducer(cycle switch) share the same 5 volt referance from the PCM.
The transducer intermittantly shorts out & pulls the 5V referance close to 0 volt.
The result is a strange running problem & a P0121 TPS low volt.
We started seein these 6-7 years ago & the first one was a b!atch to figure out.
Now it's trivial.
Thread Starter
On The Tree
Joined: Nov 2003
Posts: 175
Likes: 0
From: Pacific Beach/San Diego/US Navy,LSCM
Originally Posted by wrencher
I'll give ya this one;
The TPS & the A/C low pressure transducer(cycle switch) share the same 5 volt referance from the PCM.
The transducer intermittantly shorts out & pulls the 5V referance close to 0 volt.
The result is a strange running problem & a P0121 TPS low volt.
We started seein these 6-7 years ago & the first one was a b!atch to figure out.
Now it's trivial.
The TPS & the A/C low pressure transducer(cycle switch) share the same 5 volt referance from the PCM.
The transducer intermittantly shorts out & pulls the 5V referance close to 0 volt.
The result is a strange running problem & a P0121 TPS low volt.
We started seein these 6-7 years ago & the first one was a b!atch to figure out.
Now it's trivial.
Thank you for the reply!
Is this problem fixable?
Rx Bruce/StoreKeeper99Z28
TTT for you Bruce !
Is the A/C low pressure transducer(cycle switch) replaceable ? What's the scoop ? Hand tools ? I'll check it out on the schematic when I get home tonight but any info would be helpful.
I had the same issue with the TCS light coming on and the TPS code, but after replacing the TPS it hasn't happened again... I do still get the TCS off issue with the A/C on at idle though and I'm betting this is what it is ! It's (hopefully)all starting to make sense !
Thanks !
Bad30th
Is the A/C low pressure transducer(cycle switch) replaceable ? What's the scoop ? Hand tools ? I'll check it out on the schematic when I get home tonight but any info would be helpful.
I had the same issue with the TCS light coming on and the TPS code, but after replacing the TPS it hasn't happened again... I do still get the TCS off issue with the A/C on at idle though and I'm betting this is what it is ! It's (hopefully)all starting to make sense !
Thanks !
Bad30th
Originally Posted by StoreKeeper99Z28
____________________________________
Thank you for the reply!
Is this problem fixable?
Rx Bruce/StoreKeeper99Z28
Thank you for the reply!
Is this problem fixable?
Rx Bruce/StoreKeeper99Z28
Thanks SO much for this info !
P/N# 22664328 A/C Low Pressure Transducer/Cycle Switch (for '99 LS1)
GM Parts Counter has them for $43.84 plus tax. I'll be picking one up on the way home.
Thanks again, I'll post up if this does the trick.
BTW, I've noticed this problem to be most common among folks who have swapped cams, which normally requires the A/C system to be discharged for some amount of time (mine was discharged ~6 weeks because of an engine swap). Is it possible the time spent with the A/C system discharged could have something to do with the sensor going bad ?
Bad30th
P/N# 22664328 A/C Low Pressure Transducer/Cycle Switch (for '99 LS1)
GM Parts Counter has them for $43.84 plus tax. I'll be picking one up on the way home.
Thanks again, I'll post up if this does the trick.
BTW, I've noticed this problem to be most common among folks who have swapped cams, which normally requires the A/C system to be discharged for some amount of time (mine was discharged ~6 weeks because of an engine swap). Is it possible the time spent with the A/C system discharged could have something to do with the sensor going bad ?
Bad30th
Last edited by Bad30th; Dec 3, 2003 at 04:14 PM.
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Update for today :
- installed the new A/C sensor
- started from cold and let idle for 5 min
- turned on A/C for 10 min
- no TCS light !
Bruce we should do this for you too, I laughed when I figured out which sensor this was - it's the little sensor by the pass side strut tower in the A/C line. 5 minutes and a 14mm wrench in the parking lot at your work should have you fixed. I'll call you Monday - go pick up P/N# 22664328 from a GM Parts counter and we'll slap it in - should solve the mystery for you too.
Also today :
- drained 1 gal coolant and replaced with 50%Dexcool/50% distilled water, cleaned stupid coolant sensor with alcohol and qtips - no more low coolant light on the dash !, new radiator cap
- changed oil to Mobil1 filter and (4xMobil1 10w30) and (2xMobil1 15w50), ticking is substantially reduced and we will see if oil consumption lessens - currently burning about 1qt every 1k miles
Bad30th
- installed the new A/C sensor
- started from cold and let idle for 5 min
- turned on A/C for 10 min
- no TCS light !
Bruce we should do this for you too, I laughed when I figured out which sensor this was - it's the little sensor by the pass side strut tower in the A/C line. 5 minutes and a 14mm wrench in the parking lot at your work should have you fixed. I'll call you Monday - go pick up P/N# 22664328 from a GM Parts counter and we'll slap it in - should solve the mystery for you too.
Also today :
- drained 1 gal coolant and replaced with 50%Dexcool/50% distilled water, cleaned stupid coolant sensor with alcohol and qtips - no more low coolant light on the dash !, new radiator cap
- changed oil to Mobil1 filter and (4xMobil1 10w30) and (2xMobil1 15w50), ticking is substantially reduced and we will see if oil consumption lessens - currently burning about 1qt every 1k miles
Bad30th
Last edited by Bad30th; Dec 7, 2003 at 08:44 PM.
Thread Starter
On The Tree
Joined: Nov 2003
Posts: 175
Likes: 0
From: Pacific Beach/San Diego/US Navy,LSCM
Originally Posted by Bad30th
Update for today :
- installed the new A/C sensor
- started from cold and let idle for 5 min
- turned on A/C for 10 min
- no TCS light !
Bruce we should do this for you too, I laughed when I figured out which sensor this was - it's the little sensor by the pass side strut tower in the A/C line. 5 minutes and a 14mm wrench in the parking lot at your work should have you fixed. I'll call you Monday - go pick up P/N# 22664328 from a GM Parts counter and we'll slap it in - should solve the mystery for you too.
Also today :
- drained 1 gal coolant and replaced with 50%Dexcool/50% distilled water, cleaned stupid coolant sensor with alcohol and qtips - no more low coolant light on the dash !, new radiator cap
- changed oil to Mobil1 filter and (4xMobil1 10w30) and (2xMobil1 15w50), ticking is substantially reduced and we will see if oil consumption lessens - currently burning about 1qt every 1k miles
Bad30th
- installed the new A/C sensor
- started from cold and let idle for 5 min
- turned on A/C for 10 min
- no TCS light !
Bruce we should do this for you too, I laughed when I figured out which sensor this was - it's the little sensor by the pass side strut tower in the A/C line. 5 minutes and a 14mm wrench in the parking lot at your work should have you fixed. I'll call you Monday - go pick up P/N# 22664328 from a GM Parts counter and we'll slap it in - should solve the mystery for you too.
Also today :
- drained 1 gal coolant and replaced with 50%Dexcool/50% distilled water, cleaned stupid coolant sensor with alcohol and qtips - no more low coolant light on the dash !, new radiator cap
- changed oil to Mobil1 filter and (4xMobil1 10w30) and (2xMobil1 15w50), ticking is substantially reduced and we will see if oil consumption lessens - currently burning about 1qt every 1k miles
Bad30th
Rob;
That great!
Please call me this morning at work and we will talk.
Rx Bruce
ARGH, bad news. 
Out of curiosity I fired the AC up again today with the car warm idling in Park - TCS light came on on the dash. When the problem is happening the trans shifts harder and the car feels down on power (limp mode ?). Pressing TCS switch won't enable/disable TCS - light stays on on the dash.
As soon as I drive with the AC off for a little while the TCS light on the dash turns off and all is back to normal.
Dang, I was hoping that sensor was the answer...
Anyone have any other ideas ?
Bad30th
Out of curiosity I fired the AC up again today with the car warm idling in Park - TCS light came on on the dash. When the problem is happening the trans shifts harder and the car feels down on power (limp mode ?). Pressing TCS switch won't enable/disable TCS - light stays on on the dash.
As soon as I drive with the AC off for a little while the TCS light on the dash turns off and all is back to normal.
Dang, I was hoping that sensor was the answer...
Anyone have any other ideas ?
Bad30th
I gave the info to fix keepers car, not your tcs light.
Dont highjack threads! j/k
No but seriously your problem sounds a little more involved & alot less common.
Need a scan tool to see what TCS & ABS codes are in there to even begin to determain what is up with your car.
Then maybe you can find something in common with the proposed TCS problem & the A/C running.
Dont highjack threads! j/k
No but seriously your problem sounds a little more involved & alot less common.
Need a scan tool to see what TCS & ABS codes are in there to even begin to determain what is up with your car.
Then maybe you can find something in common with the proposed TCS problem & the A/C running.
Hehe, sorry 
- seemed like it was possibly the same problem since I had the throttle sensor issue at one point. We definately appreciate the help.
No codes are present according to AutoTap on my car when the TCS disables - but we will focus on Bruce's car here.
We're swapping the AC sensor on Bruce's car tomorrow AM (Friday).
BTW, Bruce does also have the same problem with his traction control disabling itself.
Thanks !
Rob (Bad30th)
- seemed like it was possibly the same problem since I had the throttle sensor issue at one point. We definately appreciate the help.No codes are present according to AutoTap on my car when the TCS disables - but we will focus on Bruce's car here.
We're swapping the AC sensor on Bruce's car tomorrow AM (Friday).
BTW, Bruce does also have the same problem with his traction control disabling itself.
Thanks !
Rob (Bad30th)
Originally Posted by wrencher
I gave the info to fix keepers car, not your tcs light.
Dont highjack threads! j/k
No but seriously your problem sounds a little more involved & alot less common.
Need a scan tool to see what TCS & ABS codes are in there to even begin to determain what is up with your car.
Then maybe you can find something in common with the proposed TCS problem & the A/C running.
Dont highjack threads! j/k
No but seriously your problem sounds a little more involved & alot less common.
Need a scan tool to see what TCS & ABS codes are in there to even begin to determain what is up with your car.
Then maybe you can find something in common with the proposed TCS problem & the A/C running.
Thread Starter
On The Tree
Joined: Nov 2003
Posts: 175
Likes: 0
From: Pacific Beach/San Diego/US Navy,LSCM
Yep Rob is right have have the same problem with my 99Z28 /M6.
The ASR goes off when I get on the freeway and get at a constant speed my ASR goes off..
When I exit the freeway and drive side streets the ASR comes back on?
Gotta love these computer controlled cars!
Rx Bruce/StoreKeeper99Z28
The ASR goes off when I get on the freeway and get at a constant speed my ASR goes off..
When I exit the freeway and drive side streets the ASR comes back on?
Gotta love these computer controlled cars!
Rx Bruce/StoreKeeper99Z28
You need to retrieve the ASR & ABS codes to begin to determine what the problem is.
The Tech -II can do this & a few other aftermarket scanners.
These are not PCM generated codes.
But these control units share alot of the same input sensors.
The Tech -II can do this & a few other aftermarket scanners.
These are not PCM generated codes.
But these control units share alot of the same input sensors.
Thread Starter
On The Tree
Joined: Nov 2003
Posts: 175
Likes: 0
From: Pacific Beach/San Diego/US Navy,LSCM
Originally Posted by StoreKeeper99Z28
I had my 99Z28/M6 Autotaped because my SES light came on.
The code was P0121.
It seems to happen when I turn on the Air conditioner at idle the RPM'S drop to 625-650 then it works it self back to 1025-1045 (Desired range)
A friend of mine said to Open the Idle adjutment screw. Your IAC running Warm at about 35-40.
Any help you can gve me would be greatly appreciated
Rx Bruce/StoreKeeper99Z28
The code was P0121.
It seems to happen when I turn on the Air conditioner at idle the RPM'S drop to 625-650 then it works it self back to 1025-1045 (Desired range)
A friend of mine said to Open the Idle adjutment screw. Your IAC running Warm at about 35-40.
Any help you can gve me would be greatly appreciated
Rx Bruce/StoreKeeper99Z28
Do you think because the size of the cam (233/233/578/578/113 lsa) that at idle the vacuum is low and it send that message to the Throttle Body Sensor that in turn I get the P0121 code?
Rx Bruce/StoreKeeper99Z28
Last edited by StoreKeeper99Z28; Dec 25, 2003 at 06:47 AM.
Giving thanks for this thread..... got the code and the asr light. Found this thread and realized I put some ac sealant in a few days before. Pulled the sensor and brown sealant shows itself. Cleaned it out and pushed Schrader to make sure pressure is there. Freon with no sealant blows out. Reattach sensor and test..... No more light!!!!
Thanks again
Thanks again