I think everyone sooner or later comes across this problem, when the catalyst is destroyed and second O2 sensor gives P0421 error... Also this could be a reason for higher fuel consumption and engine power loss.
All the info on the Internet about this is cr@p. Recently i have developed proper circuit, which makes ECU think that catalyst is ok. Is there anyone interested?
normally, working catalyst burns residue exhaust products, so there is almost no oxygen remaining after catalyst. post-catalyst sensor constantly shows "high" level about 0.8v which tells ECU that there's no oxygen.
now, when catalyst is broken, or physically removed (say, due to some damage), burning remaining oxygen is no longer possible, so rear sensor simply repeats front sensor signal, which is like 1-5Hz sinewave. ECU detects it as error P0421 "catalyst efficiency below threshold"
there are some tricks known over 'net to deal with that.
the most known one is putting an RC chain in series with rear sensor, in try to make a "delay" or "time shift" and make output signal relatively constant over time. this DOES NOT work. although this method can eliminate P0421 error, engine work is still wrong: tripping from "high" to "low" output level results in constant fuel:air mixture change, and all the following consequences....
other method is putting rear sensor in metal tube with calibrated hole, thus making resistance to normal gases flow, in try to physically "smooth" the sensor output. the result is the same as above: it may remove error code and extinguish "check engine" lamp, but it will definitely result in poor engine operation.
what my circuit does: it simply makes right signal from sinusoidal input this is impossible with just passive components, so an OPamp was added. also the diode in timing chain, to divide charge and discharge path of C1. other components do buffering & protective jobs. on output you get almost smoothen into straight line signal, which remains "high" all the time during L-regulation, and goes low when sensor is not yet heated (right after cold engine started) and other cases when in should be LOW. so ECU thinks there's normal working catalyst present, and stops experimenting with fuel mixture and other adjustments
Last Edit: Aug 21, 2016 8:19:31 GMT -8 by lamazoid
Lamazoid's circuit appears to be quite a useful item for the home mechanic. I would think that the greatest utility is to be able to confirm a problem with the sensor/the catalyst/fouled spark plug(s).
This circuit would be especially helpful when the "Check Engine" light comes on for older cars that do not provide OBD trouble codes. A few years ago, the "Check Engine" light on my 1991 Toyota Camry lit up and the mechanic told me I needed a new catalytic converter which was a very expensive fix. It turns out he was wrong, the Check Engine light stayed on - the real problem was with a blocked up EGR return pipe that resulted in producing high NOx levels - cleaning the pipe which I did myself made the light go off. If I had lamazoid's circuit at the time, I would have been able to rule out the catalyst and O2 sensors as the problem and saved several hundred dollars.
Clever circuit, lamazoid! Thanks for sharing your project!