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Start with finding the ac lines running through the firewall to the evaporator. Feel both of these lines. They both should be pretty much ice cold. If only the inlet line is cold and the other much warmer, you have one of three things happening. The least probable is a restricted evaporator. The next in line is a faulty a/c compressor. The reed valves inside the compressor may be going bad and the compressor is not pushing enough freon through the ac system. The most likely cause is low freon. Feel the lines 1st, this is one of the first checks I always perform.