SOLVED: Inactive o2 sensor - help needed!

Can you check that oil tem sensor and vvc solenoids have correct connectors attached following wiring from this thread:  https://www.the-t-bar.com/forum/roverlike-s-guides/99639-mg-f-tf-vvc-solenoids

I'll have a look at the solenoid connectors this evening and will get back to you.

I can see the correct/expected oil temperature in the P-Scan live scan results and it matches (more or less) the oil temp gauge on the dash, so I'm assuming that is working ok.

I changed the gauge oil temp sensor, the VVC oil temp sensor and the coolant temp sensor recently.

Looking at the plug colours it would appear I have the plugs in the wrong place, but looking at the wire colours it would appear they are correct.



 

As Bosch were the progenitors of the lambda probe & associated electronics, using a Bosch unit would give you the correct datum from which to progress.
I did some work on stoic values & collected several from my friendly scrappy, different makes of car & sensor, I was surprised how uniform most were, some were out of the arbitrary spec I adopted as a standard, one was burned out, but for the most part the rest were good.
M

surfymac wrote:

Looking at the plug colours it would appear I have the plugs in the wrong place, but looking at the wire colours it would appear they are correct.

 

Wire colours are important.

Can you unplug both VVC solenoids and check lambda sensor behaviour with them unplugged. Is it the same, or lambda sensor now shows correct behaviour.

Unplugged the solenoid plugs leaving the oil temp plugged in. Ran the car for 20 minutes to warm and connected P-Scan.

No error codes reported, temps showing as correct but the o2 sensor is still inactive and stuck at 0.49v.

Next try is the new Bosch sensor that has arrived today. I'm desperately trying to save this one as it's only got 29k miles but I may have to throw the towel in soon and break it for parts sadly.

I assume you have the polarising voltage at the sensor, this is switched by one of the relays in the engine management relay unit.
Obviously the wires from the sensor to its connector are governed be the manufacturer, the 12v wires to the loom are black (-) & blue/red (+), I can’t remember the sensing wire colour, possibly light green & grey (?).
Keep at it, it’ll be so obvious when the penny drops.
M

I'll test the wiring harness later. I seem to remember testing the heater wires previously and I was seeing 12v on those.

Is the polarising voltage from the ECU relay expected to be around 0.5v to match the mid-point of the sensor?

Do you have a copy of the appropriate circuit diagram of your car? The fiddling with lambda sensors that I’ve undertaken has revolved around my car (1998 VVC f) which had just the one sensor. Maybe my outlook is skewed with only having to shepherd one sheep 🐑, be that as it may, you will find both f & tf diagrams in the library. If, per chance your car is a Mk1 f, go to Dieters web site, where you will find an early monochrome drawing.
You will also find many things of interest on Dieters site.
M

Success!!!!! I checked the wiring diagrams and expected voltages and all was correct.

The solution was installing an original Bosch sensor rather than an OEM sensor. Straight away the O2 Sensor showed Active, the voltage is fluctuating as expected and the Fuel trim is not longer stuck at 100%.

Next step MOT, which hopefully it will pass this time as the only issue was emissions. Another MGF may be back on the road soon.....just in time for winter.

Excellent news.
Can you please tell us where you got the sensor from and the relevant part or reference number please?