Sorry I haven't had the chance to get back at you CatManD3W, I've been on the move lately. I don't really know what to suggest for you to do since I haven't fixed the CEL codes on mine. My SC have never given me any CEL codes since I've owned it until now, which is also the reason why I am now forced to do a long overdue ignition system tune-up. As for your question on hesitation, there is a thread called "definitive guid to fixing hesitation" somewhere here, I'm just not sure if its stickied, although a lot of member on here have that thread bookmarked since it is a very common problem to SC's. the hesitation issues usually go case to case, its not always the same reason that causes it. Like what m and Ali had just discussed in the past few posts, OBD1 codes are usually useless at times and you have to think and try stuff out to solve certain issues, it is not really that direct to the point but more of a hint. Just like what's going on with my issues, if you read the entire thread, I just replaced my o2 sensors, then got the code for the months old o2 sensors, then replaced them again with brand new OEM since it was still under warranty and still getting the same codes after all that. I'll keep this thread updated at all times, as I've seen a lot of old threads with the same issues as mine but the OP's never got back to give a conclusion which leaves the threads really useless and more of annoying.

 

No worries...KingPhilip...Thanks for your input...I am gearing towards swapping over the 2jzgte ECU thread that Ali has created as I plan to go NA-T in the spring....Just trying to get these annoying issues addressed in the meantime until the time comes..

 

Could it be I'm having grounding issues? I worked on changing the coolant temp sensor this evening, changed it out with a brand new ECTS. I double checked everything, I made sure the spark plug wires weren't mix matched. Anyways, I tried to clear the CEL, by removing the EFI fuse, but soon as I started my engine, CEL appears. Did it a couple of times, but still the same, CEL wouldn't clear. Unlike before, the CEL wouldn't really turn on until I reach 40 or 50 mph, actually until this morning. When I test drove the car, and was trying to lock the doors, it kept on unlocking immediately on its own? Is there any way that the codes have something to do with the factory alarm? Code 31 stands for no signal for the MAF sensor? Could that be ground? Anybody, help, please... Lol

 

catmand3w you should start a thread on your issue and post up all the relevant information. Since you have a 300 and not a 400 the diagnosis will be a little different.

Kingphilip it could have something to do with the grounds. I did come across this thread which seems to have a similar problem and some diagrams. Pin 5 should be grounded to the block and is the one that is for the maf function. the other ground wire is for the temp sensor and goes to the ecu.
So it could have something to do with that ground, I thought previously all those grounds went to the ecu but it seems the one for the maf goes to the cylinder head. definately worth a check to make sure it is in tact and not frayed, pinched, or even the terminal corroded, or worse if you had work done on the motor not even reconnected...
https://www.clublexus.com/forums/per...ad-ground.html

code 31 can come up for various reasons but I think we are getting close. also check the maf connector is in good shape and the pins are tight in the connector, sometimes those pins have a way of getting loose and not making full contact when plugged in, remove connector from the maf and push all the pins into the connector, give it the wiggle test and check for movement, also give the pins a visual for any buildup.

aftermarket alarm can cause all sorts of issues, not sure how it would trip a code 31 though if there was an issue the car also probably wouldn't start i would guess.

 

Thanks for all the help Ali, I think and I have high hopes that we're getting somewhere here. Someone from Lextreme is also working with me with my problem, and he did confirm that there is a ground wire at the back of each cylinder heads, and the one on the right side actually carry the ground for the o2 sensor, MAF sensor, and air temp sensor, and it is possible that it is affecting my factory alarm.

Little background on the alarm; I had grounding issues in the past with my power window on the driver side, which is a pretty common problem. Now, I tried to fix the ground, in the door panel, and instead of routing the ground back to the central grounding system, I decided to just ground it on the door itself since it's working. Ever since, it has affected my power locks but not the function of the alarm. Whenever I would shut the doors, the locks would unlock itself twice, but again it doesn't affect the function of the alarm itself. In other words it was just a mechanical side effect of the grounding, not sure why, but now that this is happening, I'm thinking it's because the ground of the power window and power locks is the same. What I notice with the problem with the alarm right now, is that, on a hot day, when I click on the lock button on the key FOB, after locking itself and arming the alarm, the door locks would then unlock itself when it feels like, but keeping the factory alarm engaged or armed. And when I click on the button on the key FOB again, I guess the ECU gets confused or whatnot and cannot disarm the alarm 'cause the door locks are already unlocked. It would drive fine and all, but I cannot lock the locks through the central locking button, it would either automatically unlock itself immediately or it wouldn't work at all - which confirms that the alarm is armed. Which got me thinking, maybe that's the reason why Lexus designed this car to have a central grounding system, instead of having grounds here and there, everywhere for each device. And then again, I'm amazed with how ahead this car is for its time. And even when I'm having these issues, I still love this mother****er. LOL Sorry.

Anyhow, I tried fixing the ground yesterday with my spare time, but didn't get to 'cause I didn't realize that the back of the engine is like 2 inches away from the firewall and I didn't have a tool that would reach the ground bolt and remove it. I just got one of those bending ratcheting socket wrench, and I'll find time to remove the wires and take a look at them. I'm think maybe the oil from my previous terrible oil leak problem burnt the wires insulators and exposed the wires itself causing them to short out. Judging by the sight of it yesterday, I couldn't even see the wires due to the thick grimy dried up oil covering everything there. Which also reminds me, I really need to get the engine and undercarriage detailed badly. I'm hoping to get this done very soon, as troubleshooting is very tiring.

I also had time to check the ECU this morning, it was still air tight seal, kinda gave me a hard time popping it open, but very helpful when I got it to pop open, everything in there looked brand spanking new! No leaks, no corrosion, no nothing. So that, eliminates the ECU to being the problem. I need to figure out a way to reseal it again to keep it that way too. Here's a few pics;

 

ecu looks good. so you didn't have the door lock problem before you changed the ground for the window?
I would probably fix that at some point like you mentioned but most likely it is a different problem.
see if you can even just get a wire brush back there and clean the parts of the ground you can reach and from the sides without taking the bolt off if it is to hard to get too. if that doesn't work then maybe try getting the bolt off.

 

This is crazy. I'm stressing so bad because of this. Haven't had a decent sleep in 3 nights. Lol

I've tried my best to reach that damn ground, and its just physically impossible without removing a bunch of stuff. I already dropped my socket wrench at the back and I can already smell it burning the rubber handle when I drive the car. I can feel the ground bolt, and it feels pretty tight on there. I wanna move away from the ground for now and think of something else I can eliminate from the symptoms.

I have new symptoms and it is scaring the **** out of me. I need this car running, I don't have any other car to use. I think the problem is starting to get worse. When I rev the engine on idle and let go of the throttle, and when the needle starts to go back down, it goes way low to the point of stalling the engine. It's been like this for the past 3 days now. It has no issues whatsoever when you drive it. It still runs perfectly. That's the only problems, it stalls on idle after revving the engine. It will idle with no problem, but its when you rev it that it stalls on the way back down of the RPM.

Why can't my SC issues be a simple remove and replace? Lol

 

Please make sure that if your car has AFR sensor (air fuel ratio) then you replace it with an AFR and if you have oxygen sensor then r,eplace it with oxygen sensor since signal detection circuitry inside the ECM looks for a specific voltage range for feedback signal. If the feedback signals are different then ECM will throw cel light and could also lead to damaging the ECM. Thanks

 

Haha! Didn't realize that it has been more than a year since I had this issue. I'm just going to update this because I'm so happyyyyyyyyyy! I literally just got this issue totally fixed yesterday [cause it was time to get smogged]. After doing a lot of troubleshooting to eliminate possible parts problem and issues, I knew it was either an electrical or grounding issue. I brought it to a local shop with a lot of great reviews throughout the community; Elite Service Center, which specializes in Toyota/Lexus/Scion. Told them what was going on and what has been done to try and eliminate it, they were able to move forward from where I left off and find where the problem area was. It turned out to be burnt wires from the previous oil leak problem I had that leads to the o2 sensor that's tripping the CEL. They replaced about 2 feet worth of frayed wiring harness which fixed that issue, and another one. It turns out that it was also causing my alarm to unlock itself due to the frayed wires touching each other. So no more intermittent unlocking of the doors in the middle of the night, and while driving! My SC400 is back to top notch and problem free, now it's time to sell it! Here's the pics the tech sent me of the problem:

 

So glad you posted this. I'm having the unlocking problem to and it's driving me crazy.

 

@KingPhillip where is this wire I have been having this issue with code 24 aND 31 for about a yr now on my sc300 and have done so much looking and replacing the same things you have ecu, maf, etc and no change. .maybe this is my issue as well. .what wire is that and location please..my car won't idle for the life of me and I'm depressed about it so maybe just maybe this might be the answer! !!