I have been getting the P0138 code on and off for a couple of months now. Today I decided to take a closer look. There were actually 4 codes; P0036, P0130, P0138 and a new one i had not seen before, P0420. There are some comments that the P0420 indicates that one of the cats might be bad. Can I just start by swapping out the 02 Sensors? Is there a better approach?

 

You can try to read the o2 sensors with a fast enough volt meter. They will put out a range of voltages, all of which are below one, but I forget what they are. You should see rapid movement between voltages though and it should not stay on just one. If it does, the sensor is defective. There are two or three wires on the sensor but only one needs to be read. The other one or two are for the heating element and should come up as 12 volts. This is not specific to the MINI, but more generic to all cars. Older cars (before OBD2) have only 1 wire and take much longer to enter closed loop operation. The heater is just there to get the sensor to operating temperature quicker.

One of the other tests for a bad sensor is to disconnect it and see if the car runs better. If it is giving bad readings it can screw up the fuel map. Disconnecting it forces the computer to go into open loop and run off it's default tables.

 

P0036: Generic: HO2S Heater Control Circuit (Bank 1 Sensor 2)

P0130: O2 Sensor Circuit Malfunction (Bank 1 Sensor 1)

P0138: O2 Sensor Circuit High Voltage Bank 1 Sensor 2

P0420: Converter operating below efficiency threshold.

Looks like there is an issue with the O2 sensors on Bank #1.

Generally if the sensors have some miles on them -- if you are not dealing with a sensor that is manifesting a premature failure -- it is best to just replace all sensors at the same time.

Do not clean the O2 sensor electrical connectors Some sensors "breathe" though the wire lead to the connector and cleaning the connector can contaminate the sensor. Do not touch the new sensor tips. Do not use anti-seize. (My experience with other car brands the factory sensors come with some O2 sensor friendly thread lube already present. OEM/after market sensors come "dry".)

After replacing the sensors clear the codes and road test the car. If the P0420 comes back replace the indicated converter.

I don't know MY/miles of your car but be aware in the USA converters are covered by an emission warranty that runs 8 years, 80K miles.

 

Certainly the post cat senso is bad. I would like to so the replacement myself but alittle gun shy. Tried replacing the pre cat sensor a couple of years ago. Really tight space and I could not get it to budge.

 

While two error codes point the sensor after the converter, there is one error code pointing to the pre converter sensor being bad:

P0130: O2 Sensor Circuit Malfunction (Bank 1 Sensor 1)

With other cars, my experience has been even if just one O2 sensor goes bad out of the 4. I just replace all 4. The issue with just replacing one is there is there is one new sensor and the rest are "old". The difference in age of the sensors means the fueling can differ from bank to bank. I just like to have both banks on equal footing regarding O2 sensor newness. The other issue is one replaces the "BAD" sensor. All is ok for a while then another sensor acts up. One is back under the car replacing another sensor. Also, the couple of times I have done this the engine perked up some with new sensors. Clearly the sensors had degraded.

But it is up to you of course. With a previous car I tried changing sensors but the first one I tackled I was unable to work the latch on the electrical connector to disconnect the sensor wiring connector from the harness connector. I had to take the car to a dealer to get the sensors replaced.

 

Been monitoring the codes for the past few days. Right now it is always a P0036 when I first start the car. I have always tried to do as much of the work as I can (many for fun and the challenge). I agree repalcing both is the best strategy. Appreciate the comments

 

SO picked up a couple of O2 Sensors. Was able to get the post cat sensor removed and the new one installed did not get to the pre cat one yet. When I started the Coop, I got a collection of error codes

p0037
p0340
p0031
p0445


I wasn't expecting that it would be back to no codes, particualrly after only replacing one of the sensors but these are all new ones that I haven't seen before. Car is more difficult to start but once it gets started, it seems to run ok.

 

Read a lot of the other posts regarding these codes this morning. Caught the mention of the F3 Engine compartment fuse. A quick check showed that the fuse was blown. Replaced the fuse and a couple of test indicate that the codes stay gone. Will find out after I drive it for a bit. Appreciate everyone on the forum.

 

Just completing the story. Removing the Pre Cat O2 sensor was actaully alot easier that the post cat. Using a box end wrench, I was able to get it moving without a hammer. from the bottom and then used a socket wrench to complete the removal from the top. Knocking on wood, All of the codes seem to be clear. Need a few more days to declare success.
While I was under the car, I took the time to reattach some loose heat shield to remove some rattles I was experiencing.
Take Care all
Ian

 

After replacing the O2 sensors. everything has worked fine for over a month. No error codes and I was able to pass my emmisions inspection.
Last week, the check engine light came on again. Checked the code yesterday and it was a P0420. Cleared it and today it showed up again. This is a 2004 Mini Cooper S unmodified. Is the cat bad? Issue with the O2 sensors I installed?
Ian

 

Did you ever find out the cause of this issue? I've got the same issue happening here (replaced both O2 sensors a month or two ago, and now have P0420).
And pointers would be appreciated. Thanks.

 

The catalyst may be broken or clogged.

 

That is my conclusion for my Cooper S as well. I need to replace it before July. I plan on doing it myself but leary about the bolts between the manifold and the engine on a 2004 Cooper S. I have been told that if you get the cooper up on jacks that the catalytic converter comes out the bottom pretty easily