Q: OBDI Code 25 How do I fix this? on 1991 Lexus LS400

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This code OBDI 25 is holding me up from getting smogged can someone please help. Air-Fuel Ratio Lean Malfuntion. What can cause this and how can I fix it?
(2) Answers
This code is the same as code P0171 on a later model Lexus. I wrote a code explanation to help people in the repair of this code which indicates that the car is running too lean. I do Emissions work in California and deal with this stuff every day.

here is the code explanation:
my car seams to be misfiring. i changed the spark plug wires, that wasn't the problem i put a jumper wire to get codes off dash board, it gave me a code 13, and code 25. the mechanic told me i have to change my timing belt. i don't feel that is right. is there a simple fix or something to try?