I have a 2005 Honda Civic 1.7l manual transmission with 154,000 miles that does not get driven all that often. 2 years ago had major tune up...plugs, coils, timing belt, water pump, new clutch, etc. About 2 weeks ago I replaced the upstream and downstream O2 sensors with Denso sensors because I had been getting a P0420 emission threshold efficiency code for about the last year. That appeared to resolve the engine codes and has been running fine.
Fast-forward to yesterday and I noticed that when revving the engine beyond about 1,700 rpm or so and releasing the throttle it makes a horrib