[thstone]

How the ECU reacts depends on which CEL (error) that it is detecting. My best guess is that a bad cat would not clean up any unburned hydrocarbons in the exhaust gas and thus, the downstream O2 sensor would then detect a slight rich condition. If this was the case, then the ECU would lean out the mixture. As the rich condition is continued to be reported, the ECU would continue to lean the mixture to try and correct it. At some point, the engine would likely start to knock due to running lean (you might not ever notice this low level of knocking) and then the ECU would retard the spark timing in order to avoid damaging the engine from knocking. Retarding the timing will typically reduce the power of the engine.

When you repair the cat, then the O2 sensor will report correctly and the ECU will correct the timing and mixture setttings as you drive, thereby restoring normal engine operation and power.

Alternatively, you can disconnect the battery which will erase the learned settings in the ECU and then the ECU will start up using the default settings and make adjustments based on valid sensor feedback as you drive.