MK3.5 Fault Code P0031

I have a 2010 MX5 Mk3.5 (NC FL) and as soon as I had bought it, the engine warning light came on. After asking my brother (who is a mechanic) to check it, it came up with a fault code P0031 - heated oxygen sensor. We therefore got a new one which he said was a really good brand. Not long after, it burned through that sensor and the engine warning light came on again. It was under warranty so we got it replaced. It then burned through that one so I bought another one with my own money and it’s burned through that one. Obviously, the fault is not with the sensor but lies elsewhere. I’m desperate for help. Any suggestions would be appreciated. 


Your issue is low voltage to the heater circuit which indicates a wiring issue rather than sensor failure.

Here is a very good write up on this error code together with suggestions as to how to remedy the problem. 

Given the number of sensors fitted I would expect to find a wiring fault is causing this rather than an ECU or sensor fault.

The heater circuit comprises 2 wires, both usually the same colour, perhaps white.

Hopefully an auto electrician will be able to assist you.

I hope you have not thrown any of the sensors away as in all probability there  is nothing wrong with them. Worth getting them tested either on the bench or fitted to another MK3.5 car. The heater circuit on the sensor should be easy to test and I’ll bet there is no problem with any of the sensors including the original.

IIRC when the heater circuit fails genuinely there is usually a different fault and code as goes open circuit(P0135).