So I was at Autozone picking up some plugs the other day, and they had two engine listings for my 01 Disco. Both were 4.0, but one had a "5" as the 7th digit of the VIN and one had a "2". It's the same thing if you go online to Autozone and choose your vehicle. The reason I ask is that they had more plug choices for the 4.0 with the "2" (found in 66% of 01 Discos) than the "5" (found in 34% of 01 Discos). I have the "5". They actually don't have the Bosch Plat +4 plugs listed when you choose the "5" engine. What gives?

 

I am not sure but i think thats for the secondary air on the engine.

 

I know from searching car-part.com, that if the 7th number in the VIN is '2' it does not have secondary air injection. If the 7th number in the VIN is '5' then it does have secondary air injection.

This being said...I'm unsure of what the secondary air injection is.

 

Here is what it is... http://en.wikipedia.org/wiki/Air_injection_reactor

 

Secondary air injection is a emissions control device, it injects fresh air into the exhaust to help the catalist inside the catalitic convertor get hotter to clean the exhaust better.
Tom put it in laymans terms...California emissions.