Hard downshift
Thread Starter
10th Gear
Joined: Nov 2024
Posts: 10
Likes: 1
Hard downshift
I have an 01-03 ls430 that recently started having hard downshifts from 3-2 and 2-1. The chassis and transmission have 226,000 miles, the engine has approx. 130,000. I know this is a fairly prevalent issue for the 04-06 models but upon doing research I have seen relatively little about the 01-03 models. I do know that the new engine in the vehicle has the screw-type PCV valve which I believe would indicate the engine is from an 03 likely. I was wondering if having the ECM looked at by SIA or a similar company would be the best thing to do at this point, the LS has not failed me so far and it would be disappointing to find out the transmission itself is bad. Also thought I should add the engine swap was done by the previous owner. Any and all help and advice would be much appreciated, I love this car and want to keep during it for many years to come
Instructor
Joined: Jul 2010
Posts: 766
Likes: 110
From: MI
I would have it looked at. Search the forum and Google, SIA is the most reasonable but service can be spotty. Tanin in Minnesota? is good. A west coast outfit has also had positive comments, I believe its Quantum. Their are several others, including some who list the repair on Ebay. I used Relentless in Dallas with great results but their prices have increased. Make some calls, ask about turnaround time and warranties. A very worthwhile repair.
3rd Gear
Joined: Apr 2025
Posts: 3
Likes: 0
One thing I noticed with mine is that the neutral safety switch or the prndl switch on the side of the transmission was slightly out of adjustment. This would cause a hard downshift into first. We noticed by the D light on the dash being off while in drive.
Rookie
Joined: Jan 2025
Posts: 25
Likes: 14
Mostly the ECM. It controls the engine and the automatic transmission. I have serviced so many of those LS430 ECMs that I would say that chances are 90% or more that the ECM is either the culprit or part of it. In fact, from the Lexus brand, those are the ones that fail the most when it is about the transmission. The same as for the 2001-2005 Toyota Rav4, Sienna, Camry, and Higlander that come here for service all the time.
I would suggest looking for an ECM repair shop. The circuit must be repaired and the adaptive memory for the transmission cleared, so it starts relearning from zero once the ECM is reinstalled and the car driven.
I would suggest looking for an ECM repair shop. The circuit must be repaired and the adaptive memory for the transmission cleared, so it starts relearning from zero once the ECM is reinstalled and the car driven.
Last edited by RickRod; Apr 3, 2025 at 02:35 AM. Reason: Typo
Thread Starter
10th Gear
Joined: Nov 2024
Posts: 10
Likes: 1
Update
Since initially posting I sent the ECM to SIA to have it inspected to see if it was the culprit of my downshift issue, I was told by them that it is in fact not the issue, the codes on the car are a p0420 and a p0430. I called Bell Lexus for further instruction and they told me they could diagnose the car for 200 and that the catalysts could be affecting how it runs, is there a possibility of it being a transmission solenoid despite the fact that I do not have a code? I want to be as methodical as I can be so I can refrain from firing the parts cannon
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Racer
Joined: Mar 2018
Posts: 1,282
Likes: 384
From: ESSEX, UK
P0420 and P0430 are normally indicative of a leak in the exhaust between the CAT and the downstream O2 sensor. The leak sucks in fresh air so the O2 sensors register too much Oxygen and trigger a fault code.
However, you don't know what happened to make the previous owner replace the engine, so could be CAT issues.
I would get the exhaust checked out for any leaks and fix those to get rid of the P0420/430 codes before doing anything else, as bad data fed to the ECU can cause harsh shifting and weird engine behaviour.
However, you don't know what happened to make the previous owner replace the engine, so could be CAT issues.
I would get the exhaust checked out for any leaks and fix those to get rid of the P0420/430 codes before doing anything else, as bad data fed to the ECU can cause harsh shifting and weird engine behaviour.
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Joined: May 2012
Posts: 1,136
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From: WI
We are now performing this 2004-2005 LS430 ECU repair. We also include complimentary ECU software updating in conjunction with our on-board repairs.
If you purchased a used ECU we also have the ability to write your VIN to the ECU to eliminate the P0630 VIN mismatch.
https://taninautoelectronix.com/prod...ough-shifting/
If you purchased a used ECU we also have the ability to write your VIN to the ECU to eliminate the P0630 VIN mismatch.
https://taninautoelectronix.com/prod...ough-shifting/
__________________
TANIN AUTO ELECTRONIX
262-456-4147
contact@taninauto.com
www.taninautoelectronix.com
https://taninjdm.com/ Our Lexus LS430 Dealership
TANIN AUTO ELECTRONIX
262-456-4147
contact@taninauto.com
www.taninautoelectronix.com
https://taninjdm.com/ Our Lexus LS430 Dealership
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