The low pressure switch on the silverado/sierra is usually located on the accumulator assy. This is a large aluminum "bullet" shaped component on the passenger side firewall. Will have two large lines attached. The low pressure switch will have a two or three wire plug into it and it just unscrews from the accumulator for replacement. Ensure that you get a new oring with the new switch and be careful not to cut or damage the new oring when installing. You do NOT have to recover the ac system to replace this switch because it has a schrader valve under it.
