my check engine light came on and its staying on whats the problem? not blinking on 1996 Toyota Land Cruiser

i remember that it came on a couple of months ago for awhile and i remember filling up one day with regular gas and it went off, but now its back on. i had a cat put on a few years ago. i hope thats not the problem.could it be the rear 02 sensor.

Asked by for the 1996 Toyota Land Cruiser
Need trouble codes to help you. Go to a parts house and get a free code readout.
i used a hand held dianogstic machine and the code that came up was PO420