code disappears on own?
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code 15 is a vehicle speed sensor code, 21 is O2 sensor and 32 is EGR. If your speedo is erratic or not working, I would replace the speed sensor on the transfer case. It wouldn't hurt also to put a new O2 sensor, will help you get better fuel economy. The EGR, I would leave alone until the code comes back; it could be a bad EGR transducer or solenoid, etc. Just check your vacuum lines make sure all looks good and go from there.