Car drives crazy. need some help diagnosing

Hi all,

I have a Lexus es300 1995 and lately its been acting crazy. It all started when I had to push the pedal all the way down just to get to 60 mph and when i took my foot off the pedal the car almost stopped completely, would not coast well.
Then the check engine light came on. Took it to dealer got codes p0500 and p0720…they said get a new ecm and dash cluster. Got both used from a junkyard. Car drove fine, then check engine light came on again and drove crazy again.

I was told the ecm needed no programming just plug and play …is this right?

Did I get a bad ecm? Coud it be another problem?


I don’t think that you needed that ECM in the first place.
P0500 indicates a problem with Vehicle Speed Sensor A or its circuitry.
This can cause problems with engine performance, transmission performance, and with the speedometer.

P0720 indicates a problem with the Output Speed Sensor or its circuitry.
This problem relates to the transmission’s electronic controls.

If your car is “driving crazy”, it is very possible that the transmission is in “limp home” mode as a result of bad inputs from one or both sensors.

I am guessing that the supposed need for a new dash cluster was because the speedometer wasn’t working. Am I correct? Since a failure of VSS-A would cause problems with the speedometer, as well as with engine and transmission performance, this further supports my theory that you never needed a new ECM (or dash cluster).

I would suggest that you take the car to an independent transmission shop for evaluation.
Both of these sensors are connected to the transmission case, so this is an area of expertise for a good trans shop. However, you want a GOOD trans shop, so I strongly suggest that you avoid the likes of AAMCO, Lee Myles, Cottman, Mr. Transmission, or any other chain operation. Seek out an indy trans shop that has been in business for at least 3 years, with a good reputation.

And, I would suggest that you avoid that dealership in the future.

Holy cow. Dealer service departments should be able to be liable for malpractice. Unless you can tell us the details on how they went about diagnosis.

I’m with VDC on all counts

I suggest that the power going to those sensors be checked and the signal outputs be checked also. The trouble may just be a bad connection in a connector.

hey guys

thanks alot for your responses. I forgot to mention I took it to pepboys and they replaced the speed senor that has 3 plugs. It didnt help at all…I did replace the dash as the dealer suggested…However the used ecm I got from ebay didnt fix the problem. Also Midas told me their machine pointed a new ecm.

thanks alot guys

Larry, Larry, Larry–It seems that you are unerringly directing yourself to the worst possible places for mechanical work.

While–in theory the dealership is competent, albeit expensive, your dealership is likely not even competent, based on your experiences.
And then, your other choices for mechanical work are both chain operations that RARELY have skilled mechanics on staff. I would only go to Midas for work on the exhaust system, and as to Pep Boys, I would not go there for anything except perhaps to buy some oil or some accessories.

You need a competent independent mechanic’s shop with a good reputation, rather than any of the joints that you have been patronizing. If you look at the top of this page, you will find a link for The Mechanics Files. It is searchable by zip code, and contains shops that were recommended by Car Talk fans.

PLEASE stop patronizing these chains whose mechanics are–at best–only semi-competent!

I don’t know the set up on the Lexus. But on some vehicles there isn’t “a” vehicle speed sensor. The PCM gets its speed indication from more than one - often an input shaft and output shaft sensor.

Your P0720 is for the output sensor, and it may be that a problem with that is generating the P0500. If pepboys replaced the input shaft sensor then this wouldn’t have done a thing. And it is just the kind of mistake a place like that would make.

Other that, there is still the pesky problem of wires that have to carry the info to & fro the computer…

thanks VDC. I will not be going to those places again. However, since it was a Lexus dealer I went to…they have the machine used by Lexus correct? thats what the mechanics from other places told me…when they couldnt diagnose the issue.

I am goin to try circuit board to test my ecm.

Also, I had my cel reset…reset the pcm…now its driving like a champ…for now