Where is the knock sensor located on a 2003 nissan frontier 3.3 v/6 engine?

You need to remove more than the manifold cover, I think you need to remove the intake manifold (upper and lower plenums). See the link.
2003 Nissan Frontier Knock Sensor Removal

I could be wrong, but a diesel shouldn't have a knock sensor.

Here's a link to your sensor,just enlarge picture to see. Repair Guides Component Locations 3 3l vg33er V6 Engine AutoZone com Here's what the sensor look's like. http://www.autozone.com/engine-management/crankshaft-position-sensor/duralast-crankshaft-position-sensor/nissan-datsun/frontier-pickup-2wd/2003/6-cylinders-3-3l-sfi-sc/229562_726118_0

underneath intake manifold between cylinder 1 and 2.

The pcm uses the knock sensor to monitor for engine knocking, and adjusts ignition timing based on this. If there is no knock sensor present, you will wind up with a hard fault that will keep the check engine lamp on continuously. Also, the truck will go into a more conservative driving mode to prevent knocking.
The knock sensor for a frontier is not terribly expensive, but it is a pain to access on nissan v6s, under the intake manifold

it is located on the bottom section of engine, Just unplug it and it screws out fairly easy

the knock sensor on a 3.5L should be just under the exhaust ports on the right side of the engine if you're facing the side with the pulleys and crank(front) for the life of me, I cant remember if the engine is mounted longitudinally or tranversely in the frontiers.....lol. There should be a wire going to a small black thing thats just kind of there,... its really easy to miss. if you can, check the nissan forums or somewhere online the e-manual that should have the exact placement of it in diagrams.

po325--ignition system or missfire.

Knock sensors can be difficult to locate because there usually buried under intake or exhaust manifolds and misc. engine wiring and plumbing. Knock sensors need to hear an engine knocking noise, therefore there usually located somewhere they can hear an engine knock. If it's a V6 then possibly it will be between the cylinder heads and access to them can only be attained by removing the intake manifold. I would suggest calling a Nissan Dealer parts person and ask them. Good luck.

knock sensor is located on the block usually under the intake. nissan has problems with these sensors. i have chaunged plenty of these on nissans in the past year. this has no affect on your gas mileage. this sensor picks up misfires on the engine. sometimes you might have rough idle and can't feel it in the drivers seat but the knock sensor can. i would suggest maybe a tune up spark plugs, air filter, fuel filter first before going to take apart a bunch of things to change a sensor. the sensor is designed to pick up any differences in motor quality and will throw the engine light on. sometimes the problem is the knock sensor, or there is an underlying cause that the knock sensor is reading.