Installed an L-shaped spacer (3 x 3 inches) at the second O2 sensor on a 2000 Forester, to lay to rest the P0420 code that has been coming on weekly for the last five years! Now I'm getting a P0136 code. My understanding was that the O2 sensor distal to the cat served no critical purpose, other than to warn of poor emissions. But some P0136 code explanations suggest that the ECU may interpret P0136 as a lean fuel/air state. Is this true for a 2000 Forester? Should I shorten the spacer to avoid a P0136 call? Both original O2 sensors were replaced with new ones a couple years ago.