My check engine light comes on every once and a while and when I run the code, i get code 2-2-1 and nothing else. Any idea what it is or how to diagnose what it is? Can I test the AMM sensor? Also, there's an oily substance in the intake hose which is kind of concerning. it seems to be coming from a hose connected closer to the intake manifold.

 

The Volvo 7xx/9xx FAQ says 2-2-1 is... Fuel trim (lambda control) too lean at part load, and possible causes are air leak; low fuel pressure; bad AMM

https://www.volvoclub.org.uk/faq/EngineOBDCodes.html

 

how do I find out which it is?

 

start by looking for intake air leaks: inspect vaccuum lines tears/cracked tees etc. inspect the intake air boots - you can spray carb cleaner around to see if it gets sucked in and bumps the idle.

 

you said its only happening occasionally, does it correlate at all with having used heavy throttle, like accelerating up a grade in a middle gear?

what model/year volvo is this, you didn't mention? turbo or not ?