2002 Chev S-10 Blazer - Replaced Intake manifold gasket and =now I have a check engine Light - Crank sensor problem - why would this come up now and what can I do to fix it

The air-fuel ratio sensor is located in either the exhaust manifold or the front exhaust pipe. It measures the amount of air in your exhaust and uses that information to determine your current air-fuel ratio.

Best to you

Check the intake manifold gasket. If it leaks it will draw air into engine and this air bypasses the mass air flow sensor which will cause a rough idle or running symptom from running too lean. If you change the intake manifold gasket you should probably change the valley pan gasket at the same time. A dealer will change both for less than $500 and you might save some more if you dig around on GM Service website and find a coupon discount (Usually 10%-20%). You might save more at a local shop if you trust them and they do this type of work. Getting to the intake manifold gasket also exposes the valley pan and since you are already there it only takes a little more to change that gasket too. Some GM V-8 engines use a little oil between oil changes and if both gaskets are meticulously changed at the same time it will sometimes fix the oil usage. It did for me on my Tahoe (126k miles).

Use one labelled all makes/models (such as one available from Prestone). You can also use DexCool.but it breaks down fairly fast (2 to 3 years).

Hi, the code 171 indicates a leak in the intake manifold gasket. You can check by getting some carb cleaner and spraying it on the intake manifold gaskets whith the engine running. If the engine revs, you have a leak. The code 420 indicates the front catalytic converter is worn out, but I would fix the vacuum leak first and see if both codes clear. Please let me know if you have questions and thanks for using FixYa.

do you mean the valve cover ? because I don't think you can have the intake manifold backward.(if it's backward, you cannot connect the upper hose and the fuel lines)
so any way, it sound like you have the valve cover swapped. it won't hurt to drive but it is a problem to fill the engine oil as you stated.

Check all vacuum lines to the parts that were removed

Change your idle control sensor, but first make sure your throttle linkage isn't sticking.

That normally means that you have a issue with your gas cap try replacing your gas cap first then diconnect the negative terminal of the battery for about three minutes to clear the memory and start the truck again

the chev are fames for intake manifold seeping. it will have to be replaced soon good luck

spray carb cleaner on the intake manifold near cylinder 1... common problem is the intake manifold gasket leaks vacuum causing a misfire...the not starting could be due to a faulty crank sensor