jdsande3

OK, I bought a 98 accord 5-speed 4 cylinder and have a CEL coming on with a code of P0420. The catalytic converter has been previously replaced at some point with a universal aftermarket weld on cat. It is hard for me to tell if the rear O2 is OK, bad, or just a bad cat or both. I know the downstream O2 should be a steady line, but it seems to be dropping out (indicating its bad), but doesn't look to be following the upstream O2 so maybe the cat is good? Are both bad? Another issue is sometimes, the car after reving (or pushing in the clutch coming to a stop) will stall. Sometimes its perfectly fine, but I do not know if the two are related.

Freeze Frame Data for address 10
PID02 DTC that caused freeze frame data storage: P0420
PID03 Fuel system 1/2 status: Closed loop, using O2 sensors
PID04 Calculated load value: 58.8 %
PID05 Engine coolant temperature: 90 °C
PID06 Short Term Trim - Bank 1: -6.3 %
PID07 Long Term Trim - Bank 1: -5.5 %
PID11 Intake Manifold Absolute Pressure: 60 kPa abs
PID12 Engine RPM: 2230 /min
PID13 Vehicle speed: 93 km/h
PID15 Intake air temperature: 47 °C
PID17 Throttle position sensor: 20.0 %

The green trace is the downstream O2, and of course, the red is the upstream O2. The upstream O2 sensor is brand new. Data taken at idle.

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MAD_MIKE

From the two screen shots when the Primary O2 dips and holds so does the secondary O2.
When there is normal switching on primary, the secondary stays fairly constant.

If the car is stalling there is something wrong that needs to be fixed. I don't think the O2s are faulty.

Have you checked your IAC? If it is dirty/faulty there is no way for the ECU to control dashpot settings and the car will stall after a rev. I would start there first.

jdsande3

Thanks for the help. I will check the IAC valve, it does not surge, but only stall. As you say, I'll start there.

DCFIVER

OBD2 vehicles are extremely finicky when it comes to CATs. If the replacement catalyst does not have the same amount of noble metals as the OEM cat,you will get a P0420. Sometimes the code will set immediately,other times it may take up to a year or so. From the graph you posted and the datastream, the vehicle appears to be in fuel control,so at this point Id call it a bad cat.