Best Way to Earse CEL History

Other day i was doing alot highway driving and got about 16 miles per gallon. This is bad mileage for my vehicle.
I have a cheap scanner i usually clear O2 CEL with and usually can go 300-400 miles on one tank without another CEL coming on.
Yesterday I pulled ECU fuse out for 10 min and put back in started engine back up and CEL was still on. Not really sure how thats possible but it happened.
So what is best way to clear not just light but entire history from the computer ?

 

might be a "hard code" and cant be cleared...what code is it giving?

 

What code(s) is it setting?

Unhooking the battery clears codes and adaptives but it's not gona fix your problem.
Need to see what code(s) it's setting to know what to start looking at.

 

As a test I unhooked the MAF sensor (easy to get to) and then took out the ECU fuse in the fuse box next to the battery, so in my test it set off some MAF codes not sure which exactly. However the code it keeps throwing is P0420 and i think its because theres a downstream O2 sensor that has a dent in the shell that the wires go into.

So in general is unhooking the battery better way than taking ECU fuse out ? Because now i almost want to unhook the main connectors from the computer and just leave it unpluged over night in case theres a capacitor or something that needs to completely drain to reset everything.

 

I believe if you want to truly clear it out, you need to disconnect BOTH battery connections and hold them together for...????10-20min???
like so....

 

Providing there are no exhaust leaks near or upstream of the AFR and O2 sensors 95% of the time a P0420/P0430 is a failed catyletic converter.

Need to verify the AFR and O2 sensors are working properly but odds are it's the converter.

https://youtu.be/Oivzxq_RuAk?si=3MbB_g4tSDOIikQH

Unhooking power from the PCM wheather pulling the fuse or unhooking the battery clears codes and all adaptive memory.

 

If your scanner use to be able to clear the code but now can't most likely the problem has gotten so bad that it is immediately causing a problem when you start the truck back up. Time to actually fix the problem.

 

Yes, my CATs got removed all 4 of them. Im using sprak plug foulers to try to keep light from coming on at first it worked but then i noticed it came back and then i noticed the upper part of the downstream sensor had a dent in it right near where the wiring converges and goes into the O2 sensor.

 

Are you running anit-foulers on both the upstream and down steam O2 sensors or just the downstream? Should only be on the downstream. If you have it only on the downstream then you might need to replace the O2 with the dent, may be a pinched wire. I'd try having it scanned with a better scanner with live data to see what exactly is going on with the O2 voltages.

 

I unhooked the MAF sensor and made sure code was on.
Took and put a wire onto the battery cables (with the cables disconnected of course). Let it sit unhooked up to cables for several hours and then reconnected the battery and the MAF related code I triggered on purpose was gone. At the very least it cleared the code as good as my basic code scanner does. Will update after drive a good distance if the gas MPG is back to normal and there still no codes.