I have a 2000 Montero Sport. Got code PO421 and have sequentially replaced all 4 O2 sensors in addition to having a complete tuneup including a new timing belt. After replacing the 4th and final O2 sensor I thought all was well but 2 weeks later the same code reared its ugly head. Where do I look from here, could it be my cat? The car seems to run just fine.
Friday, December 15th, 2006 AT 7:31 PM