I changed the one on my '97 Hardbody at about 105K, a few months after I bought it. It was still flipping between lean/rich/lean/rich during the specified timeframe in the service manual - but I was getting less than 20MPG and I had already replaced the plugs, rotor, cap, wires, fuel filter, etc. Changing it got me over 20MPG. If you are monitoring your gas mileage closely, I would just change it if you notice an unexplained drop. I have found the OE direct replacments at Spark plug, Spark plug wire, spark plug gap, spark plug cross reference and ignition wires at Sparkplugs.com by NGK and Denso to be excellent and are truely "direct replacements" for a much lower price than OEM.
I agree on changing the front (upstream) one - it's the one that the ECU uses to control everything. The rear one is just to make sure the cat. is working.
Heath
I agree on changing the front (upstream) one - it's the one that the ECU uses to control everything. The rear one is just to make sure the cat. is working.
Heath
