low output voltage in SMPS

I am experiencing similar to this person in the following thread.
Circuit board has 2 outputs 24/12. But both register 18/8 on voltmeter. Should I start with replacing the capacitors in output circuit?
My circuit board is LG, number 5800-p5l031-w000

Which rail is used for the regulation? - usually though the non-regulated rails increase if the capacitor on that rail fails.

Sorry. Should have mentioned that. Regulated one is 24.

justuju said:

Sorry. Should have mentioned that. Regulated one is 24.

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Do you have a schematic?, if the 24V is the regulated one, and the capacitor on that is high ESR, then the 24V should be about right (that's what regulation does) and the 12V is likely to be considerably high (such faults, across numerous devices, often cause serious damage, as all rails other than the regulated one are much too high).

many SMPS boards use a TL431 on the cold side as a voltage reference in series with the LED inside the optoisolator. if the TL431 is shorted, it will reduce the output voltage proportionally. if it's not a TL431, then a zener diode or string of zener diodes (usually all 6V zeners because they have near-zero temperature coefficient).