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Paul M ASE Master, ASE Master Tech
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Lexus GS 300: I have a 1998 lexus gs300 with obd code P1128

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I have a 1998 lexus gs300 with obd code P1128 and P1129. The throttle control motor malfunction. I've replaced the throttle control motor, the TPS and the accelerator lever assembly. I've checked the wire connection and have cleaned and resintalled them with no change. The car is in limp mode and I can't get above 30MPH. I don't know what to check from here except the wires from the control motor to the ECM but I don't know which ones. I found the green/red stripe one and that makes continuity. The brown/red stripe I could not find but it did make continuity to the pink and black wires one the end of the ECM back plug.
Thank you for choosing JustAnswer and allowing me to help you resolve your Lexus questions! Did you replace the Throttle motor/ body assembly with a new or used one?
Customer: replied 4 years ago.

All parts replaced are new OEM.

Customer: replied 4 years ago.

I've also checked for any binding of the throttle plate. Cleaned it up with brake cleaner...etc. I want to check the wires before trying to replace ecm. That is a last resort. The parts already have cost close to $500.

Do you have the part numbers of the parts that you have replaced?
Customer: replied 4 years ago.

The throttle control motor part number is XXXXX /p>


The throttle lever assembly i got from toyota 22060-46010. The TPS I got from NAPA but it is not showing a problem. The OBD code is P1128 and P1129. I had P1120 and P1121 but resolved those.


Ok now why does this matter ? What are you getting to ? I've already informed you that I've replaced with OEM parts. I have to work on this ASAP. If we are just going to exchange one question a day, I ask that we either move faster or get someone else.

Customer: replied 4 years ago.

If you have access to the wire information from the ECM to the throttle control motor harness, that would help me debug this. The check engine light comes on instantly after clearing it. I suspect it is wiring and that one of the wires is shorted to GND.

Customer: replied 4 years ago.

the throttle control motor did not show on last reply. it is XXXXXXXXXX sorry about that

Customer: replied 4 years ago.
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Hello I'm Paul and I will help you with an Answer to your Question

Hello I am attaching the diagnostic information so you can check the wiring harness and ECM


click here to view diagnostic info for p1128

Click here to view p1125 (referenced in p1128 diagnostic)

click here for diagnostic info for p1129


click here for the wiring diagram of the ECM


click here for the ECM pin locator information

let me know if you need more help


Customer: replied 4 years ago.


I've checked the resistance of the throttle control motor between pins 1 and 2 and it is about 25 ohms, so that is with in the spec. correct ? I checked each pin/wire from the motor harness to e2 connector (31P) of the ecm and each wire checks out (green/red (20) and yellow/red (19) and black (7) of connector E2 )EXCEPT the braided wire (30) reads about 500 ohms so it does not or seems like it does not have continuity. Is the braided wire not connected or shorted or is that the correct resistance ? If not how can I replace the braided wire or is it the ECM ? I've cleaned, and checked for any mechanical binding of motor and throttle plate, throttle body.

Customer: replied 4 years ago.

Why is this closed ? I have not received a response as of yet. my email is [email protected]. Thank you for the schematics. I did some testing and replied with my results and other questions but the question says it is closed.

Did you also check the White 8 (E2)?



the Braided wire is a shield and is not included in the diagnostic testing.



25 Ohms is within range and all 4 listed in the diagnostic check out with 1 ohm or less


this points at ECM but you need to check the grounds before replacing the ECU


click here to view the test info and scroll to the part for CHECK AND REPLACE ECU


remember to rate my answer so I get credit for my help if you need more help just let me know the quesiton will remain on the site for follow up.



Customer: replied 4 years ago.

The white 8 E2 checks out ok with continuity. The gnd pin according to the last document you sent about checking and replacing ecu does not show low resistance. It is very high, can't read it. I checked from the black wire 4th pin on the bottom from left to right. pin 28 of e2 from other document. This doesn't show continuity or less than 1 ohm. So can I tap into that wire and ground it to the body ? I checked the connector and it appears to make good contact and the gold pin itself is clean and not bent or anything wrong. It says it is rare for a wire to be broken in between so what would it be ?

The last link is an example on testing ECU grounds


the black at pin 4 is just an example and does not apply to your ECU


you need to find your grounds using the Engine Performance wiring diagram I attached above


I went through and did this for you... here are the grounds to test


E2 (9) brown
E3 (17) brown
E3 (1) white/black
E2 (31) White/black
E2 (21) white/black
E7 (22) whte black
E7 (9) white/black

if the grounds check out suspect ECU is faulty



Paul M ASE Master, ASE Master Tech
Category: Lexus
Satisfied Customers: 4684
Experience: ASE Master Tech 20+ Years
Paul M ASE Master and 2 other Lexus Specialists are ready to help you
Customer: replied 4 years ago.

Well all the gnds check out. They all have continuity to body gnd. The motor checks out ok. The throttle plate and body don't have any binding. The wires in the motor harness to ecu check out ok. So I guess I have to replace the ecu. I will be checking online. Thanks for your help and time in resolving this quickly. Previous guy was taking too long.

Customer: replied 4 years ago.

Hi Paul, I asked a question last month regarding my lexus gs300 with P1128 code.
I've replaced the Throttle motor, the TPS, The accelerator lever assembly motor(the one on the back of the throttle body) and I used your diagrams and checked out all wires from the harness to the ecu including the gnds. I purchased a used ecu and had it reprogrammed. I'm getting the same result and it is now codes P0120, P1125 and P1128.

if p1125 was not coming up before there may be an issue with the ECU or the connector


only 3 possible causes are listed for p1125


1 Open or Short in the Throttle Control Motor Circuit


2 Throttle Control Motor




im curious why the new ECU is pulling up 2 new codes

did you clear and recheck?

did the old ECU show p1125? is it a factory replacement? can you install the original ECU to see if p1125 is still present?

click here for details on p1125



Customer: replied 4 years ago.

ECU is factory replacement and programmed for theft system. I dont' know if 1125 was on old ecu or not. I will check it out. when I first plugged in new ecu, it showed pass on code reader and the check engine light was on. The codes finally showed up after awhile. I have checked the wiring and ohmed out the throttle control motor last month and all checked ok. Both motors were good less than 25ohms. I did notice that the entire throttle body and throttle control motor get very hot...I don't know if the cooling system to the throttle body is plugged or not. I don't have any overheat problems and I had put a new radiator cap to be sure of flow. just wondering if it gets hot would that throw a code. I attempted to clear the codes but they will not clear not even once. They do not show up in pending codes either, just fail I had checked continuity on the harness using the diagrams you gave me last month and at that time they all checked out ok which is why I got to this point of replacing the ecu. Now I'm at a loss since I got an ecu and have the same problem. I've spent almost $1000 now in parts and programming. labor has been all mine except the programming.

Customer: replied 4 years ago.

I put old ecu back in and got only code 1128 (no 1125) erased it but light still on, then I put new ecu in and got only 1128. So it is now consistent for both ecu's. I cleaned the wire harness's on the car and contacts on both ecu's. Then I tried both again BUT this time I got 1129 on both ecu's. I am now leading to believe that neither ecu is bad and that yes it is in the wiring or connector somewhere. can you resend the diagrams so I can check them out ? they were only temporary and I cant access them anymore. OR what else do you suggest ? both 1128 and 1129 point to throttle control motor. I ohm'ed the new and old one and they check out ok (within spec).

Customer: replied 4 years ago.

is code 1129 related to the TPS ?


Throttle Control Motor Lock Malfunction


Electronic Throttle Control System

Diagnostics for p1129 are saying to replace the ECU and if code is still present replace throttle body.

click here to view information for code p1129


very odd your getting a new set of codes now but this is what I have on them


click here to view info on p1128


the old links are also up again


Customer: replied 4 years ago.

Ok I will be checking this tonight when I get home from my day job. I did have codes 1128 and 1129 from the very beginning. Then it went to just 1128...after replacing the ecu it showed 1128 on both ecu's, now it has 1129. I hope this is it. Do throttle bodys go out ? It's got all new parts that are attached to it. There isn't much left except for the throttle blade/valve. The entire lever assembly is new oem, tps and throttle motor are new too. I've also cleaned it up and checked for any binding. But I will have to find a throttle body and test it. nothing else to check right ?

Throttle Body , Wiring and ECU are the 3 possibilities.

I would have thought changing all the electronics on the Throttle body would solve this personally but most tech info advises to change the entire unit.

I would probably go through all the wiring tests a second time also

Customer: replied 4 years ago.

I was not able to get throttle body just yet so I went through the wiring a second time. I did find one problem with E7(22) white/black to GND. It did not have continuity. I tapped into it with another wire and connected it to E7(9) white/black and then it had continuity to body GND. BUT the car did not start, the anti theft was flashing, the red light for the key. When I took it off then it started. So I tried the E7(22) straight to GND and it was the same, so I took it off then car started. Same errors of course. P1129. So if that is supposed to be connected to GND why wouldn't it work? How do I repair it ? This may be my problem all this time.

I double checked the pin 22 of E7


it goes to the transponder for the key is not a ground

Sorry for the confusion it should be E6 (22) not E7 (22)


E7 (9) is a ground



Customer: replied 3 years ago.

Ok I checked all GNDS again and the E6(22) is GND ok. So i'm not able to get a throttle body at this time. I'm also confused why the documentation says to replace it. Anywhere I check the COMPLETE throttle body is all they have for sale, not the core. I've replaced everything that is attached to throttle body, TPS, Throttle control motor and the lever assembly. So it's maybe under the impression that if I were to replace the throttle body it would include all the parts I've already replaced. Not saying you say this...just by what the documents recommend. Anyway don't have the funds for it. So I took the throttle body off and cleaned it up spic and span. I noticed a small cover plate on the top and removed it and cleaned the very small pinholes that are in there. Would that help ? The throttle plate and body are clean now. I will try that, if no go, then I may have to buy the throttle body but I'm afraid it may not fix it. What about fuel pressure or lack of ? would that cause it ?

Fuel pressure would not trigger these.

hard to believe these codes are still there with all electronics new and wiring tested.

you might want to closely look at connectors and try wiggle testing them

Customer: replied 3 years ago.

I will try wiggle test and wiring again. The wiggle test is hard to do since I can't use the lever assembly to full throttle it. How can I wiggle test ? Just go through each of the wires and see if they are loose? making a good connection ? The ECU pin locator info. document does not load. I want to check the wiring for the accelerator lever assembly. The lever assembly only allows slight opening of the throttle plate. Is that the way it is setup? It is drive by wire right ? So the throttle control motor takes over from there ? The plate does not bind or anything just asking because the throttle lever does not allow full throttle.