Lambda Sensor again

 While I was celebrating the MOT pass I was hit with lambda problems again.   As regular readers will know, a failure of the sensor is almost unnoticeable as the car performs normally.   But purely by coincidence I decided to do an instrumented run.   The log immediately showed the problem, in that the lambda sensor voltage was firmly fixed at 435mV and the whole journey was in open loop.

Although I had rewired the system a few months ago I really didn't like my solution so thought it was time to rip it all out and start again.  So the first step was to rewire the heater from where it left the loom.  Before plugging in the sensor I checked that the voltage output was zero until you started the engine then it jumped to 13.5V as the alternator kicked in.

Next I replaced the signal wires with 2 separate screened cables, one for the signal and one for the earth.  Probably overkill, but I happened to have  some single-core screened cable I could use,

So all plugged in I started the engine and plugged in MEMSScan to monitor the lambda voltage.  Unfortunately it stayed at 435mV, although after 3 mins or so it gradually crept up to 450mV, so something was obviously still wrong.

Not sure why I did it but I retested the  heater wires unplugged and found they were still showing 13.5V,  but when I plugged in the sensor the voltage dropped to 0.11V.   So offload it was OK, but under load it was failing.   Next step was to swap the sensor for my spare, but that didn't help and the same happened.

I needed to get her on the road so as a short-term measure (although I am the first to admit my short-term fixes have a tendency to become long-term)  I provided a new feed to the heater using a dedicated earth and an ignition switched 12V.   The downside to that is that the heater is activated immediately you turn on the ignition rather than waiting till the engine is running.   I just have to make sure she doesn't sit with the ignition on for very long.

MEMSScan appeared to show that now everything was working.   So took out for a long run and here was the log file:

 Happy days 😁

So I still don't know why the voltage dropped dramatically when the sensor load was added, I need to investigate that further.

I also now know that the sensor will NOT work unless the heater is on, even when the engine has warmed up to temperature.