So I bought my first disco a few weeks ago (04 disco SE with SAI) , and the mechanic told me he replaced the MAFS (Bosch) as well as both upstream O2 sensors. (Bosch). Well i got it home and hooked it up to a computer and reset the adaptations and cleared the codes. It ran great! Well the service light came back on for bank one upstream sensor, and heater circuit P0135 and P0134. I replaced the sensor with a new Bosch from amazon and reset the codes and i did not drive for 5 min when they came back on for the same thing, but now with P0505. Could there have been two bad sensors in a row or something else? The connections look clean. Bank two is working fine when viewed on live data. However every so often it will throw a bank 2 code? Can anyone help or at least guide me to check further.

THANKS,
Firefighting_DiscoII

 

There are a lot of threads on this forum documenting people's problems with multiple Bosch O2 sensors being dead out of the box. Walker brand seem to do well, and I believe there's another brand that is also much more reliable.

 

When I buy the walker ones should i change the pair or just bank one?

 

Walker sensors are not always but often Denso repackaged. Denso sensors are pretty good. On my BMW, which is like a BOSCH temple, I went with Bosch and had the same issues with the new ones. So I went with another brand and all was well.

 

On my Disco 2 parts list for O2 sensors I have:
- NTK (NGK) #25623 UP & #25624 DOWN or Walker #350-34462 UP & #350-34188 DOWN; Denso maybe ***NOT Bosch!***

I'd change both O2 sensors in pairs if you don't know how long your O2 sensors have been on your Disco. They might be the original O2 set installed from the UK Solihull factory 16 years ago. lol

 

The first post says that the upstreams were both replaced. However, I would still replace them as a pair. Since they're newly installed, it should't be difficult to get them out.

 

I chased this same issue for a few months with a Discovery I had just purchased.

My final conclusion was there are three components:
O2 sensor
Wiring
ECU

To help diagnose the issue you need a device that reads the engines live data so you can see the voltage from the O2 sensors live. I used Bluedriver that plugs into the ODB2 port and is controlled from an app on your phone

Most likely an O2 sensor right out of the box is going to be good. Switch the front O2 Sensors to confirm this since your codes are for the left bank.

After that and your sensor is good check the wiring at the connection for the O2 sensor (mine had severed)

If your wiring is good, find a salvage yard or ebay and get a matching pair of BCU and ECU and swap them for yours. (tip here, after you install them you will have to put the key into the II position for about 5-10 minutes until the red light goes off on the instrument panel. This is synching issue the BCU has with an electronic part that is built into the interior fuze box)

After this if you see the live data being good then you have a decision to make about the instrument panel. You could install the instrument panel that came with the BCU and ECU so the computer's milage in the BCU and instrument panel match. If you don't the instrument panel milage will blink. You can turn this blinking off with a Nanocom, the milage will be wrong for what the actual milage of the vehicle is. The Nanocom is supposed to be able to synch the milage of the BCU to a different instrument panel as long as that instrument panel is lower. (so it will bring up the milage to match the BCU, The BCU can not bring down milage on the instrument panel). Although when I tried this on mine I could not get it to synch.

Mine is a salvage vehicle so I don't have the issue of the milage not matching so now I have an instrument panel that does not blink but is about 5,000 miles lower than the actual milage on the vehicle.

 

Having same issue. x2 bad O2 sensors out of box. Now a third! Brand new cats. My downstream bank 1 stays at .7 no matter if the opposite side is cycling as it should, and the inline upstream one reads less than .7! unfortunately, have a bosch replacement on the way..... will have to try denzo....

 

When at operating temp I never had a downstream sensor give a cycling reading unless a cat was failing. They should read steady.

 

Interesting, though they should always be less than the upstream one and cycle more slowly overall....maybe it's the other downstream one that is bad...
and if it's correct and staying at .7 system too rich....