Catalyst monitor & Oxygen sensor monitor not readay

Hi! I had previously dtc p0420 (Catalyst Converter BAD) on my 2002 Nissan Sentra GXE (1.8L), so on advise of the mechanic got the Catalyst converter replaced. Since, O2 sensors were fused with the old Cat, hence had to get them replaced too. Now, for inspection maintenance, both Catalyst monitor and Oxygen sensor monitor show NOT READY status. I try to work on the car drive cycle to get them to READY status following the owners manual. While at the drive cycle, dtc po153 (Oxygen sensor 1, bank 2) which reads “oxygen sensor circuit slow” comes up. I got the oxygen sensor replaced twice already, but it comes up everytime. Only good thing is that Oxygen sensor gets ready even though it has that dtc but Cat doesn’t get to ready status (may be it first needs NO dtc on O2 sensors). So, I am finally stuck at this circular problem. My mechanic also seems to be at his wits end. Any feedback will be appreciated.

First I would get the correct DTC code(s). While it is correct that P0153 does refer to an O2 sensor problem (circuit slow response) for bank 2 sensor 1, your car does not have a “bank 2”. There is only 1 bank of cylinders on your 1.8L. So P0153 references a sensor you do not have.

There are two oxygen sensors. One in front of the CAT and one behind…The CAT itself is judged by the difference in readings between the two 02 sensors…When you install aftermarket components like sensors and CAT’s, sometimes the readings seen by the engine computer are different from what it has been programmed to expect. It starts throwing codes because it thinks something is wrong…I would go to a Nissan parts counter, buy the two (front and rear) oxygen sensors, install them and see what happens…

I would like to amend my previous statement. Since looking at an actual part diagram I see the 1.8 in your car does indeed have 4 o2 sensors. Nissan lists them as Oxygen sensors 1, 2, 3,and 4. So that code may well be correct and I just don’t know how it relates to this engine. The code may refer to cyls. 1& 2 as bank one, and Cyls. 3 & 4 as bank 2 I really don’t know. As Caddyman said, usually on an inline 4 cyl there are only 2, o2 sensors, pre-cat (upstream, sensor 1) and post-cat (downstream, sensor 2). My apologies.

Does the car have dual exhausts? Things could get complicated…

Thanks to both of you Caddyman & PvtPublic for your comments! I got started with your comments and did a little research on the problem. Nissan Sentra 2002 GXE has a configuration SIMILAR to SEMI DUAL exhaust. SEMI DUAL exhaust systems have 2 CATs for 2 banks of cylinders & after the CATs the exhaust join through a Y type pipe to pass through a single muffler to the final tail piece of the exhaust. By SIMILAR to SEMI DUAL, I mean the exhaust from 2 cylinder banks pass through a SINGLE CAT but it that SINGLE CAT has two channels in it. So, Yes it’s a SINGLE CAT from outside, BUT DUAL CAT from inside. By its functioning, for simplicity, we can safely call it SEMI DUAL Exhaust. However, the exhaust system has another downstream CAT, which is secondary. The downstream CAT acts like a SINGLE exhaust with just one channel in it. That downstream CAT has been behaving well so far :slight_smile: As far as I understand, it is not even connected to the PCM.

Now, by BANK 1 sensors, all it means is that the sensor is located in that channel of the CAT wherein cylinder 1 of the bank is located. So, for my car, it would be the LEFT sensors (looking from the REAR of the car). Even Nissan OEM and popular sensor vendors like NTK use left and right notation. Coming to Sensor designation 1 & 2, it’s the upstream and downstream of CAT respectively.

Caddy’s point is well taken. BOTH 1 & 2 sensors need to be from the same manufacturer (QUALITY) and both better be NEW as it’s the difference in readings between the sensors that matters. As sensor gets old it’s response to lean/rich fuel mixtures slows down or amplitude gets less and less. If the one in the downstream is new and the other in the upstream is old, there is a potential for messed up readings. As you can see, I have improvised a little on Caddy’s point:) That’s for now. Thanks again for your comments! I will keep you updated on the progress.