I have a 98 accord that will frequently post a P0420 cat code. Looking at the freeze frame data the LTFT3 and STFT3 both indicate -100.6%. Is it possible that the fuel trim is causing the P0420 to post??? If so, any ideas why such out of bounds fuel trims??
Freeze Frame Data

Throttle Position 18.4%
RPM 2031
Load Value 60.7
MAP Sensor 62 KPA
Coolant 197degs
STFT1 -5.1
LTFT1 -1%
STFT3 -100.6
LTFT3 -100.6
Speed 52 MPH
Fuel System 1 Closed

 

i wouldn't think so. this code is for no good catalyst efficiency. it's based on a differential between upstream and downstream O2 sensors (or lack thereof.) i suspect you would see a P0171 or P0172 if there were truly a fuel trim problem. it's odd that it's off like that but if only P0420 is showing, you should only troubleshoot that. no other drivability issues?

you could confirm it's not just an intermittent fault by clearing the code, bringing the engine to operating temp then driving for a few minutes at 45-50mph. lift off for about 3 seconds (throttle completely closed). if the code returns, replace the cat.

 

Thanks for the reply... I have cleared the code several times and it always reposts. I removed the O2 sensor in the CAT it had quite a bit of soot on it. The tailpipe also shows soot. I did replace the O2 sensor in the CAT and am waiting to see if it reposts.

 

yes it can cause the code p0420.

 

Please exlain.

 

Thanks, Any ideas why such a high negative fuel trim??? No real driveability issues, only a slight hesitation some times.

 

I may need help understanding the scan tool here... THis is a 4 cylinder engine, should there only be 1 set of LTFT and STFT?? THe scan tool outputs 2 sets of fuel trim values for LTFT and STFT (LTFT1, STFT1, LTFT3 and STFT3). Is the LTFT3 and STFT3 from the scan tool bogus if this is a 4 cylinder motor???

 

Your problem is either the oxygen sensor or cat here. Not sure why you're fiddling with long and short term fuel curves.