I am pretty sure the lambda probes on my 1995 Softdash classic are dying. They seem to read .45v with very little change. The car itself does not have a steady idle, and when on gas the mixture control just goes up and up till it stalls.
I have a pair of three wire sensors, but have read something about the sensors used on the classic being an odd type, I’m unsure is this is true as they seem to behave just like standard 0-1v sensors, which is how the LPG system was setup to deal with them, the only oddity being they need a 17mm spanner not a 22mm like normal ones

I've read that the three wire need special grease to earth them. As the four wire have an independent earth wire. Yours don't.
https://autoditex.com/page/lambda-sensor-o2-sensor-21-1.html
Is this any help at all?

Can you read any live data on your classic with diagnostic software at all?

Mark Piercy said:

Can you read any live data on your classic with diagnostic software at all?

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Yes. Although it is through the LPG diagnostic, it sits at a steady 0.45v most of the time, then flicks briefly to .9v then back to 0.45v that is followed by a miss from the engine while running on petrol. Happens once every 6-7 seconds

Zaphod said:

Yes. Although it is through the LPG diagnostic, it sits at a steady 0.45v most of the time, then flicks briefly to .9v then back to 0.45v that is followed by a miss from the engine while running on petrol. Happens once every 6-7 seconds

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OK, if kept on petrol they should rise and fall. This allows the cat to get a gob full of oxygène to burn off the pollutants. The pré cat observés the engine and the post cat observes the cat.
They help the fuel injection system richen and lean off the fueling by affecting the fuel trims.
If they don't rise and fall then they could be at fault.
By introducing a separate gas/fuel into the inlet side of the engine the fuel trims will change because the lambdas detect a rich condition. They will jump up and the fuel trim will reduce the petrol input and then the reading will settle somewhat to a rise and fall pattern. When the gas/fuel is stopped the lambda reading will read a lean condition and the fuel trim will jump up and then settle again. Can you read the fuel trims on the ecu?

If the lambda doesn't react to a gas/fuel introduced then the sensor could well be knacked!! Are the plugs and wiring in good tat?