in case anyone was wondering I have a d900 as well. It happened today. I installed my draxas header/flex a couple weeks ago and got a light, no code though. The other day I put my stg1 cranes in and started running rich enough to smell gas from the tailpipe. Today I plugged my new xcal in and my CEL went away. Then I went into test mode and noticed it was throwing a d900!? now I have no CEL, d900, and gas smelling exhaust.
P2270 - Lack of HO2S-12 Switch, Sensor Indicates Lean
Description: The downstream heated oxygen sensor (HO2S) is forced rich and lean and monitored by the powertrain control module (PCM). The test fails if the PCM does not detect the output of the HO2S in a calibrated amount of time.
Possible Causes: Pinched, shorted, and corroded wiring and pins.
Crossed sensor wires.
Contaminated or damaged sensor.
basically it's saying that your downstream 02 sensor. The one after the cat. Is bad, or your catalytic converters are not doing their job. Or in the case of having a race header, the converter is non existant.