ECM (Computer) in Default Mode?

1998 K-1500 Z71
V8 Vortec 350 5.7-L
125,000 miles

My truck continues to drive poorly. I replaced the catalytic converters two weeks ago thinking they were clogged or malfunctioning. The new cats made no difference to the driveability. I'll describe the symptoms again: My truck has, on most days, poor acceleration, lack of power, and the gas mileage runs lower than what it should. But, on other days, the truck runs like it was brand new, with the horsepower and awesome acceleration that it should have. What is causing this very noticeable difference in driveability? I was reading my Chilton manual today and I read the section about the ECM (Computer). It said that when the ECM is bad the computer will go into a default mode and the driveability will be noticably poor. These are the conditions I am experiencing on most days. But if on other days, my truck runs like it is supposed to, could there be an electrical short somewhere between the ECM and power source? Has anyone experienced this problem? How can I get the ECM tested to see if it is bad?

Does the check engine light come on? Whenever the ECM goes into "limp" mode, it is suppose to turn on the light.

Have you scanned for codes when it is running poorly?

//2000CAYukon

Yes there is code that is showing right now. It reads P0430. From my understanding, this code says "catylist efficiency below threshold in bank 2" which indicates an O2 sensor is not reading. I took my truck into the dealership to test the O2 sensors and they were fine, however, the cat on bank 2 needed replaced. I replaced it. But for the past 6 months or so, my truck has experienced the driveability issues as I described in the original post. When the truck is driving poorly, no light comes on.

Has anyone else experienced this?

You've described my symptoms on my '94 Isuzu Rodeo to a tee. Chiltons tipped me to the ECM default mode also. I don't get an engine light and haven't checked codes. Bought at a good price couple of weeks ago because prior owner basically gave up after replacing lots of stuff from fuel pump to coils, wires & plugs and most of the sensors - but not the ECM. Before spending the $300 or so to replace or repair, does anyone know how to test the module or who might be able to? Any help would be appreciated.

Have the ignition coil tested autozone well do this for free

welcome to the auto forum. this is not the correct location for your vehicle issue. go to the correct forum for your vehicle.

with a vehicle at this age electrical issues and poor grounds do cause erratic engine operation. the ECM does get damaged from poor grounds. signals do get distorted and may cause the computer to go to default / limp mode.

if you do get a refurbished ECM it must be programed for your vehicle so when ordering it make sure they have the correct info.....