Q: My 4-cylinder S10 1999 failed on Cadillac converter slightly too warm not red.. it passed everything else.. no check engine light

"Bad" is a relative term when it comes to oxygen sensors. It is possible for the oxygen sensor to slowly degrade and lose effectiveness over time but not so much so that the check engine light comes on. However, oxygen sensors can be tested using even inexpensive (DIY) scan tools such as the Bosch OBDII 1200 (or higher). You can graph the output of the sensor and also view the switching voltage range to see if it is functioning properly. What you want to do, with that or a similar scan tool, is also look at the fuel trims. If fuel trims are out of range, that’s a sign of poor engine running condition and will adversely affect emissions. Catalytic converter effectiveness is monitored by the downstream oxygen sensor and, as well, you can simply measure the in and out temperatures of the converter to see if it is actually working. The outlet temperature should be higher than the inlet. If your vehicle burns oil, that is something to consider both in terms of emissions and converter longevity. If you have never replaced the valve stem oil seals, that will help reduce oil consumption. The bottom line is you will need a diagnostic, and scan tool, to determine what has to be optimized to reduce emissions. If you desire that, please simply request an engine diagnostic and the responding certified mechanic will get this taken care of for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.