SC400 Code 28

[skirts]

I have a code 28 on my 95 sc400. I have had the main oxygen sensor replaced twice and the light is still on what should my next step be? The only reason I haven't been the one replacing it is because the sensor was under warranty from the previous owner having it replaced. So Im going to swap the sensor to the other bank to see if the code transfers? Any other ideas??
It seems like weather affects it also, on cool mornings the light may never come on, but when its hot out the light is on when I start it and stays on.
Thanks Lextreme.

 

[skirts]

I swapped the Oxygen sensors last light and I still have a code 28???

 

[kingphilipii]

Hi guys! Sorry to bump a dead thread... I'm a regular at CLub Lexus but no one can seem to help me with my problem. I went through the exact same thing the OP experienced. I have a '95 SC400 with 198xxx miles.

I replaced my main o2 sensors about 6 months ago with the hopes to fix my gas mileage issues, then I got CEL for code 28(main o2 sensor right bank) a month ago. Got the o2 sensors replaced with brand new OEM Denso since it was still under warranty. Then did an entire ignition system tune-up with it. I replaced spark plugs, spark plug wires, distributor rotors and caps, main and sub o2 sensors - all with brand new OEM Denso/Toyota; and did an oil change with Seafoam to it. I didn't overdo Seafoam as I've heard horror stories about people who overused it. I simply did what the instructions said, and divided it into 3, poured 1 into the gas tank, 1 into the oil crank case and another one into the intake through the PCV hose.

After everything, I still get the code 28. This time accompanied with codes 24(Intake Air Temperature Sensor Circuit) and 31(Air Flow Meter Circuit), which I later fixed, I don't know how, but after checking all spark plug wires connection to the spark plugs and distributors, and cleaning the battery terminals and putting everything back together the codes disappeared.

Car is running fine, no problems whatsoever, no hesitation, no stutter, and idles perfectly. I've done the 12volt mod in the past - maybe a year ago, MAF sensor is fine and passes tests, coolant temperature sensor has been replaced about a year ago with a Duralast brand from Autozone. I also noticed that the CEL does not turn on immediately, unless the car has just been shut off. It doesn't seem to be engine temperature dependent, wether I wait for the engine to warm up to normal operating temperature or drive it before it warms up, the CEL wouldn't turn on until about a mile or so of driving. It seems that it does run a bit rich, but not as bad as before the tune-up, based on the smell of the exhaust. My gas mileage is slightly better as well compared to before the tune-up when I used to get anywhere from 13-15 mpg of city driving, now I am getting roughly 16-17 mpg city. I tried to input every single detail so somebody can better determine my problem. Thanks in advance for the help.

 

[kingphilipii]

Hi guys! Sorry to bump a dead thread... I'm a regular at CLub Lexus but no one can seem to help me with my problem. I went through the exact same thing the OP experienced. I have a '95 SC400 with 198xxx miles.

I replaced my main o2 sensors about 6 months ago with the hopes to fix my gas mileage issues, then I got CEL for code 28 a month ago. Got the o2 sensors replaced with brand new OEM Denso since it was still under warranty. Then did an entire ignition system tune-up with it. I replaced spark plugs, spark plug wires, distributor rotors and caps, main and sub o2 sensors - all with brand new OEM Denso/Toyota; and did an oil change with Seafoam to it. I didn't overdo Seafoam as I've heard horror stories about people who overused it. I simply did what the instructions said, and divided it into 3, poured 1 into the gas tank, 1 into the oil crank case and another one into the intake through the PCV hose.

After everything, I still get the code 28. This time accompanied with codes 24 and 31, which I later fixed, I don't know how, but after checking all spark plug wires connection to the spark plugs and distributors, and cleaning the battery terminals.

Car is running fine, no problems whatsoever, no hesitation, no stutter, and idles perfectly. I've done the 12volt mod in the past - maybe a year ago, MAF sensor is fine and passes tests, coolant temperature sensor has been replaced about a year ago with a Duralast brand from Autozone. I also noticed that the CEL does not turn on immediately, unless the car has just been shut off. It doesn't seem to be engine temperature dependent, wether I wait for the engine to warm up to normal operating temperature or drive it before it warms up, the CEL wouldn't turn on until about a mile or so of driving. It seems that it does run a bit rich, but not as bad as before the tune-up, based on the smell of the exhaust. My gas mileage is slightly better as well compared to before the tune-up when I used to get anywhere from 13-15 mpg of city driving, now I am getting roughly 16-17 mpg city. I tried to input every single detail so somebody can better determine my problem. Thanks in advance for the help.

 

[gloverman]

single wire sensors or 4 wire? Have you scoped the voltage of the sensors to monitor what they are reading?

 

[kingphilipii]

It's 4 wire. It's brand new OEM Denso. Can you enlighten me on "scoping the voltage?"

 

[gloverman]

It is connecting an oscilloscope to them to see if they are working ie correct voltage and cycling up and down as they should. There is connection points in the engine diagnostic port.

 

[kingphilipii]

I don't have an oscilloscope. Is there another way to see? Btw, the codes 24 and 31 are back. And my factory alarm is going coocoo on me. There are times when I push the lock it will immediately unlock on its own and pushing the lock the second time wouldn't work anymore. I'd have to insert the key in and switch it to on for the locks to function normally again. Does it have anything to do with ground?

 

[smokeyo1]

Earth it and earth it again, short and sweet

 

[gloverman]

I would be checking the AFM unit then considering the ECU having issues. We have replaced/rebuilt 6 over the last month. The Air temp sensor and air flow meter is basically two units housed together so unless there are wiring issues to them. Wiring sometimes gets burnt when it falls onto the exhaust at RHR of the engine and oxygen sensor wiring is back there too. If you open the ECU and look at the base of the caps you may see corrosion.

Less time on the internet and more time working on the car is the best answer

 

[kingphilipii]

Less time on the internet and more time working on the car is the best answer

Just a brief history of the car and myself; I bought it around 3 years ago, and has been my daily ever since. It was parked by the previous owner for roughly 3 years as well. I dealt with every common problem these cars have. First year I owned it, I fixed a lot of suspension problems, steering, and leaks. I have changed every single bushing this car has, I have replaced the power steering pump, rack and pinion and alternator a few couple times. This car has never had a CEL ever since I've owned it until now. It doesn't show any kind of problem, no hesitation, no idle issues, it has never stalled not once. Every single problem and issue this car had was done and fixed by me and only me. No shop or dealership have ever touched this car ever since I've owned it, other than getting an alignment and getting tires installed. Every single mod was also done by me and only me. With all due respect, I wouldn't be here in this forum right now if I don't work on my car. I would've just brought it to the dealership or a shop if I don't work on my car. I am here right now because I am at a complete lost and don't know where to start looking for the issue my car is having. I am not a trained mechanic, I am just an enthusiast that loves his car dearly. I've searched high and low for answers, I've read countless threads here and there, in every forum Lexus or 1UZFE related. I really appreciate the help, but no help is worth it in exchange of arrogance. Thank you.

 

[gloverman]

I was meant as humour not arrogance. Sorry If I offended you in any way I am sorry , It was in no way my intention.

 

[kingphilipii]

Ok, thank you for your help, I really appreciate it. If you could assist me further, I'd like to eliminate every other part that can cause these problems first before getting in to the ECU. Just wanna make sure that my car is still mechanically able. So far I've replaced these parts, all with OEM Denso;

Spark plugs
Spark plug wires
Distributor rotor and caps
O2 sensors main and sub
Coolant temperature sensor

I have a K&N air filter, just cleaned and oiled.
I did an oil change and oil filter change with Seafoam on the old spark plugs.
I've set the TPS to spec less than a year ago.
I've changed the fuel filter in the last year.
I've done the 12volt mod on the fuel ECU.
I've checked and listened to the fuel injectors with a stethoscope to verify their function, and all is firing/clicking in timing and consistently.
I have 3 MAF sensors, all passing volt and resistance tests, that I've been using simultaneously and have never given the car problems.

Is there anything else I'm missing? Is this all a sign of ECU failure or malfunction? My car passes smog with no problem whatsoever. I wanna eliminate that the car is having grounding issues too, since my factory alarm was kinda acting up the past couple of days. I've had a very bad oil leak in the past year, and the engines back and undercarriage is badly covered with dry grimy oil, will this cause problems with wiring or grounds? I've scheduled my car to get an engine and undercarriage detailing in the next few weeks, to get a better look at the wirings and parts. If it is so the ECU, is there another reason it failed but does not show problem in the way the car runs other than being a gas guzzler(15mpg) which code 24 is triggering to go on safe mode?

 

[gloverman]

Ill try and find a photo of where the ECUs give the main issue. Its really easy to pop the lid off and have a look. I have had them with leaking caps and they run fine and drive fine just like yours but throw random codes. One simply didnt have an engine check light , otherwise completely fine.

Watch for too much oil on the air filter then sticking onto the MAF.

The main engine earths are on the back of each cylinder head. A little tricky to get to but just take your time. The looms do fall to pieces from years covered in oil.

I would be checking that Right hand rear of the engine loom where it flicks around the back of the engine. In that loom is one engine earth , the airflow meter , air temp , ignitor and RH oxygen sensor. Im wondering about this as you are getting air temp, RH oxy and airflow meter codes and are suspecious about an earth. It runs directly above the exhaust and I have seen quite a few that have needed a repair there.

Is there any chance you might have a local workshop with a scan tool that can check some live data? Even better let you have a look. Just to see what the ECU is seeing. It is really hard to find such a workshop who actually know what they are looking at. Ive just started teaching some of my local workshops how to use their scanners to work out different issues. Its mostly good thinking processes and thinking.

 

[kingphilipii]

Alright, so I tried working on the ground this afternoon, it was quite a tight fit, I didn't realize how close the rear part of the engine was to the firewall. I did find a couple of bolts to the engine head, but I cannot identify which one is the ground, due to lack of lighting, and the rear part was just heavily covered in grime and oil. I just need a bit of help finding out which is the ground in the picture? Is it the one in the red circle or blue or is there another bolt I am not seeing here? Is there an easier way to remove the entire wiring or do I need to pop the valve cover off for easier access?

Forgive my questions if they seem stupid, I'm currently looking for diagrams in my shop manual as well as I'm typing and I couldn't find a better quality picture in the net. I'm going to pop open the ECU as well tonight when I get more time in my hands to inspect it for corrosion and leaking capacitors. Thank you.

 

[kingphilipii]

I just got done checking the ECU for leaks and whatnot, not that I'm bragging, but being an Electronic Engineer, everything looks fine in there. No leaks, no corrosion, actually the ECU was still sealed air tight, and was pretty hard to pop open and everything inside looked brand spanking new. Now I need to reseal it properly to keep it that way, 'cause I kinda damaged the cover trying to pop it open. Anyways, I just got my self one of those bending ratcheting socket wrench so I can remove the ground at the backside of the engine.

 

[gloverman]

ECU is good news.

The blue circle is not the earth. It holds the plastic bracket in place. Blue circle may be the earth. Im trying to find the home camera cord so I can put up a photo. I happen to have three Soarer looms in my garage.

 

[gloverman]

Photos. Earths are the ring terminals. One on the plastic side at the bottom of the cover. The piece of plastic around the side has been broken so it looks a bit funny. Other Earth is beside where the trans loom goes down the back. This is a JDM soarer loom which is RHD so there might be a bit of a difference but all these engines have earths in about the same spot. Earths can be checked with a multimeter as well.

 

[kingphilipii]

I'm having a hard time reaching the ground at the back of the engine head, it's pretty tight in there. Is there a better way to reach it? Should I remove the heater core valve in place? Will I damage anything doing so, like gaskets or seals?

I drove the car this morning about 30-40 miles back and forth, then rested for like an hours or so. When I started up the car again and turned on the AC to leave, I noticed my idle spiking in the low 1k and down around 200 RPM. It idles better with the AC turned off, but when I rev it and let go of the throttle, the idle dips so low until it stalls. It doesn't stall on its own, only after revving the engine. I drove it around a couple miles away, haven't checked back on it, but it drives really well, like usual, and I also noticed the CEL is going on and off, for no particular reason. Any help is still appreciated, thanks in advance.