My kid owns a 1999 Miata MX-5/NB. Recently it's begun lighting the Check Engine light. I checked the codes with my OBD2 scanner and it's logged P0140 with P0171 pending. I cleared the codes but they returned in a few days.

I know that the P0140 refers to the second oxygen sensor, and I believe this is located between the catalytic converter and the muffler.

I have a few questions:

1) Am I right about the location of this?
2) Can it cause engine damage if he drives it for a few weeks without fixing it? I'm assuming that if this code is real, that probably the engine is running too lean or too rich, but it sounds normal. (We've had the car for about six months.)
3) Could this be caused by a loose hose, something causing a vacuum leak?
4) Where does the wire run from the ECU to the sensor -- is it behind the driver's seat?

I'm asking that last question because we installed a backup camera in this car over Christmas, and we hid the cable running from the display (on the dashboard) to the camera under the upholstery behind the driver's seat. I saw another wiring bundle under there but we didn't disturb it, so far as I know.

Thanks for any help.

 

What diagnosis have you carried out for P0140 & P0171 to confirm the fault is with the sensor & not elsewhere?

People confuse the Air/fuel sensor as another oxygen sensor.

The A/F sensor will be in the exhaust manifold, the oxygen sensor will be either in the side of the Cat or post cat.

The A/F sensor measure the fuel left in the exhaust gases and the Oxygen sensor measures the oxygen levels coming out of the Cat.

P0140 is more than likely a dead 02 sensor, where as P0171 is a indication of the engine running too lean, which could be a fuel restriction, air / vacuum leak post Air Flow meter, or a faulty A/F sensor sending incorrect data to the PCM. Monitoring Short term & Long term fuel trims will aid in the engines current fueling conditions.

I hope this helps, as in most cases, a fault code only indicates that the results read by the PCM/ECU are outside a prescribed parameter under certain operating conditions.