2001 DTS, 4.6L northstar

A few months ago i started having surging and RPM at idle rapidly varying from around 300 to 1500. My local mechanic changed the O2 sensors, but i started having the same problem about a week later. The latest error code reads something like "camshaft position sensor" and another reads "camshaft sensor circuit malfunction". I just bought a replacement camshaft sensor to install. Is the sensor relatively easy to replace? Is this a common problem with this engine model?

Also, i have read about some problems that occur from the crankshaft position sensors being bad and a lot of them sound like the same problems i am having although im not getting any error codes regarding the crank. Could my crankshaft sensors also be bad even though im not getting any error codes?

Any advice would help. Thanks in advance.

Camshaft position sensor problems aren't common, but the unit is on the belt drive end "front" of the engine, in the face of the rear cam cover, just below and rearward of the power steering pump. You'll see a squarish unit with wire pigtail bolted to the front of the cam cover. It takes some doing to get to. If the connector appears oily and dirty, you might just get away with a good cleaning and reconnect.

Crankshaft Position Sensor (CKP) problems are more common, but generally do not cause surging idle: they cause low-speed coast-down stalls. If one is bad it sets the timing to a default retard setting and sets codes P0335/6 or P0385/6. There are two, located on the front side of the block, just to the right and above the oil filter adapter. The upper one is B, the lower, A. Old units made in Mexico, replacements Japanese. Different p/n's. www.rockauto.com . Also not the easiest to get to. You really need a hoist for them.

A surging idle sounds more like a vacuum leak somewhere. Have your mechanic use a tad of starting fluid to test for a vacuum leak around the 8 intake manifold gaskets and also under the rubber bellows/plenum which connects the throttlebody to the intake manifold. Do a throttlebody cleaning. Many posts in here on the subject. Use TB cleaner, NOT choke cleaner. Check the EGR valve for proper operation.