In March I had my catalytic converter and both oxygen sensors replaced. Last week (June) my check engine light came back on. I got one of those wifi OBDII plug-ins and ran tests via the Torque App after clearing the CEL. After driving around for a while all tests were OK, then suddenly one switched to fail--Catalyst Monitor Bank 1 (MID:$21 TID:$a0). Minimum test value is .35, but Current value was registered at .324. What does this mean? What should I do to fix it?