2.3L ('83-'97) Code P0420

[mdiiorio]

Hello,
I have a 1995 Ford Ranger 2.3L and am getting code P0420 “Catalyst System Efficiency Below Threshold Bank 1.” I first noticed it last summer especially on really hot days as a pending code and it would intermittently appear on my diagnostic scanner and then go away, but it never triggered the check engine light until now. About a year and half ago I replaced the upstream O2 sensor to fix another code and it was effective, but the downstream sensor was not replaced and it is the original. I’m think it’s either the cat converter or the downstream sensor, in your experience what does this code usually indicate? The fact that it would intermittently come on and off as pending for a whole summer makes me think it was the cat converter gradually getting more and more clogged. Could it still be the o2 sensor? I’m no mechanic.

Thanks, Mike

 

[RonD]

Its good to look up OBD2 codes on a full list, like this one: https://www.therangerstation.com/tech_library/OBD-II_codes.shtml

Computer has these to chose from
P0420 Catalyst System Efficiency Below Threshold (Bank 1)
P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)
P0422 Main Catalyst Efficiency Below Threshold (Bank 1)
P0423 Heated Catalyst Efficiency Below Threshold (Bank 1)
P0424 Heated Catalyst Temperature Below Threshold (Bank 1)

AND....it has these O2 sensor codes to chose from, Bank 1 Sensor 2, is downstream O2
P0136 02 Sensor Circuit Malfunction (Bank I Sensor 2)
P0137 02 Sensor Circuit Low Voltage (Bank I Sensor 2)
P0138 02 Sensor Circuit High Voltage (Bank I Sensor 2)
P0139 02 Sensor Circuit Slow Response (Bank 1 Sensor 2)
P0140 02 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
P0141 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)

So for P0420 Catalyst System Efficiency Below Threshold (Bank 1)
My money would be on Old Cat converter
If the downstream O2 was failing, and it might be, I would expect an O2 code
Its cheaper to replace the O2 and if its old probably needs it, but as said I think the Cat is failing

You could swap O2s around, put the new one at the back and move old one up front

 

[mdiiorio]

Thank you. I’ll probably start by replacing the old o2 sensor and if that doesn’t work I’ll replace the cat converter. If I went straight to the cat converter I would replace the o2 sensor anyways. Thanks!