1993 SC400, with 130,000 miles. Only modification is a cat back x pipe with no mufflers. I’ve been getting code 21, and 28. When you first turn the car on no code. The CEL comes on after about 3-10 minutes of driving. I’ve now replaced my o2 sensors with 2 new Bosch upstream o2 sensors. I’m still receiving codes 21 and 28. These are the only codes. Is there anyway I have a pre cat exhaust leak? Or could something else be causing an issue? I know some say it could be the ecu, I am trying to eliminate all other options. Thanks in advance for any help!

 

Check to make sure your wiring hasn't been damaged. I think these ECUs are more with idling issues and AC problems.. I'd check the wiring going to the ECU to ensure they weren't modified previously.

 

Here's another earlier thread that may help you as well. You may be right with the ECU, but I'd check wiring first as previously stated.

https://www.clublexus.com/forums/per...-and-28-a.html