Obdii-trouble Codes

The P0136 code refers to the upstream O2 sensor, or pre-cat O2 sensor. Doesn't necessarily mean a bad O2 sensor - just that the ECM detected a lower than expected voltage from the sensor over a specified period. Possible causes are a short in the wiring, bad chassis ground, if the O2 sensor was recently replaced - could be an impedance mismatch (very common when using a non-OEM universal or aftermarket sensor). If this is the original O2 sensor and you've noticed either a steady decline in fuel economy (lazy or dying sensor) or a sudden drop in fuel economy (dead/dying sensor) - might be time to replace it.

The P0171 is probably related to the P0136 - as when the engine runs lean, there will be a higher oxygen ratio presence and the sensor will report a lower voltage. dorman68 makes a good point of trying to clean the MAF sensor first (in the airbox). Not usually a maintenance part, but many have been able to get good results with careful cleaning. After cleaning the sensor - reset the ECM and see if the codes come back. If they don't come back - probably was a dirty MAF sensor. If it comes right back - then check the wiring/connection to the O2 sensor before you replace it. Could be as simple as a corroded connection under the passenger side carpet, right next to the console.

Ground is just about any metal on the car. Quick check for ground loops - using a high impedance multimeter set to millivolts, touch the negative battery terminal to various metal parts in the engine bay/body - should read close to zero volts everywhere - if you are picking up any significant voltage (10s to 100s of millivolts) - then you have either a ground loop(s) and or corrosion between the battery "ground" and the chassis "ground".

If I remember the colors on it, the O2 sensor should have 4 leads on it - 2 black (Heater + and -), blue (signal +), and white (signal -) on Denso OEM. Might have to pull back the carpet in the front passenger seat to get at them. The white leat and one of the black leads should read zero volts when referenced to ground (pick a good solid metal point in the car). If you are really careful, you can also check the resistance between ground points. I tend to not recommend that, as if you accidentally resistance probe a component that is particularly sensitive - like the O2 sensor signal wires, or squib for the seat belt pretensions/airbag, etc. - would blow something up or short the sensor out. Making a resistance measurement - the multimeter will apply a known voltage across the probes. Making voltage measurements, a high impedance multimeter is essentially invisible to the circuit at hand.

I would look at any vacuum leaks from the airbox to the throttlebody. Some good places to check are the top airbox cover (sometimes doesn't fully seat into the tabs in the rear) and the o-ring on the MAF sensor itself, make sure it is in good shape, lightly lube, if needed, with silicone grease or something that is safe for rubber/plastic materials. Make sure the airfilter sits well inside the airbox, nothing should be sticking out. Possible that the hose clamps are too tight (crushing the piping) or too loose (piping too sloppy), could be cracks/cuts in the 90 degree elbow piece. Any of the vacuum hoses running to the intake manifold and throttlebody are other potential sources. Throttlebody gasket could be bad, intake manifold gasket could have a leak.

Possible that excessive oil flung onto the sensor "cooked" the MAF. You can try running an OEM paper airfilter temporarily and see if the code comes back. If it does, then I would suspect a bad MAF sensor. See if the parts store has a return policy on the sensor, incase the new sensor doesn't solve the CEL (most stores will not except returns on sensors, unless defective). I personally like OEM sensors (O2, pressure, temp, MAF, etc.) - but many have used a reman or aftermarket one and gotten good results. Shop around for them - I've heard that some have gotten them for $60.