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DTC P0121 after FAST install....???

This is a discussion on DTC P0121 after FAST install....??? within the External Engine forums, part of the LSx Technical Help Section category; I installed a FAST 78/78 set up (tb and intake). I drove it about 200 miles or so, then I ...

  1. #1
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    Question DTC P0121 after FAST install....???

    I installed a FAST 78/78 set up (tb and intake). I drove it about 200 miles or so, then I get 2 codes. I get the 121 and 507. I had them erased by a Tech 2to see if they would come back.

    I drove another 10-15 miles, the SES came back on. It stayed on for about 110 miles, and then I stopped to get gas. I left the gas station, and I looked at the dash, and the SES was gone??

    I get back home, and plug up the Autoscan just to see if there were any codes still there, it was showing the 121 again, but no SES light on the dash???

    I looked it up in a little more detail and it supposedly means:
    P0121 sets when the predicted throttle angle does not match the actual throttle angle.
    P0122 sets when the TP sensor signal voltage is less than 0.2 volts.
    It sounds like you have a wiring problem ( broken wire somewhere). You can count on this low voltage being lower than normal closed throttle voltage. A bad ground would increase sensor output so rule that out. It is possible that your 5 volt reference voltage took a dive so carefully inspect the wiring to the sensor. To save weight, the wiring is smaller and is more prone to breaking inside the insulation. The telltale signs of internal breakage would be a bubble in the wiring insulation. You could also have had a loss of 5 volt reference from the PCM. If so, you would expect DTCs from other sensors using the same 5 volt reference. Usually this is an EGR DTC.

    I found that info on CZ28.com Can anyone help me out a little?

    On the FAST tb, it has 2 places to hook the throttle cable up to. I could only get it to work in the first hole. B/c when we hooked it up to the second hole, the throttle was part way open, and there was no way to lengthen the cable.

    Can anyone explain some of this to me? And it has affected the power big time IMO. It starts out great down low, but when I go WOT, it bogs for a second or two, like I am running on 4 cylinders. But when it hits the 3k RPM range, it picks up like it should.

    I would appreciate any help on this. I am assuming it is a FAST quality issue, and I don't think tuning would fix it. I think it is the stupid FAST tb. I thought about putting the stock tb back on to see if the issue went away.

    Thanks for any info.
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  2. #2
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    Did you order the FAST TB with the TPS, or did you reuse the TPS from your stock TB?

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    Quote Originally Posted by rbrack View Post
    Did you order the FAST TB with the TPS, or did you reuse the TPS from your stock TB?
    I reused the stock TPS. It said in the directions that the stockers could be switched over.

    I put the stock TB back on last night, w/ the sensors, and drove around. NO problems at all. The car was back to normal, it was like gaining 50 hp, and driving on all 8 cylinders again.

    I would be willing to try the sensors that are an option to come w/ the FAST tb. If you think that is the problem. But if the new sensors were mandatory for proper fuctioning, shouldn't it say that somewhere?

    Where can I get just the sensors?

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    I am sure this is not what you want to hear but many people have said the FAST throttle bodies are not that great and have had issues like yours and with them binding. I have the 90 MM intake and like it but got a Nick Williams TB. If you just recently got the TB I would call the place you got it and tell them the problems and see if they will either exchange it for another TB or let you get a different one (manufactuer) all together.?
    98 T/A: HPE 5.3 Heads, 238/242 113 cam, Kooks 1 7/8 stepped LT headers, Custom Y pipe, Cut out, Borla, NW 90MM TB, FAST 90MM Intake, TB Bypass, Under Drive Power steering and Crank, 150 Dry NOS, Yella Terra rockers, 3400 TC, HPE tuning,

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    Quote Originally Posted by Transam388 View Post
    I am sure this is not what you want to hear but many people have said the FAST throttle bodies are not that great and have had issues like yours and with them binding. I have the 90 MM intake and like it but got a Nick Williams TB. If you just recently got the TB I would call the place you got it and tell them the problems and see if they will either exchange it for another TB or let you get a different one (manufactuer) all together.?

    I agree, I had read some things about that. And I found out that the TPS and IAC sensors are the same that you can get at AZ or the dealer. And since I put the stock TB back on, and it worked GREAT, I am blaming it on the TB.

    I sent an email to the dealer I bought it from. Let's see if he lives up to his good feedback. I bought the FAST TB on 8/31/06.

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