I recently bought a 2004 Suburban 1500LT with 250,000 miles. The guy drove it around for a few months with the service engine light on. It's throwing codes PO136, PO137, PO156, and PO157. Is it a safe bet that all four oxygen sensors are bad? It's quite possible they haven't been replaced in a long time, and It's not throwing any other codes. Also, is it correct that all four sensors are the same? I don't have a whole lot of experience with a Suburban or the 5.3s. Thanks!