Regulated Low voltage signal creator
Here's a suggested circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

Now for a quick explanation:

U1 is a voltage regulator. It outputs a fixed voltage of 5 V, regardless of fluctuation in the battery voltage. This ensures the output voltage stays fixed regardless if the battery voltage is 11 V, 14 V or anything in between.

R1 and R2 form a voltage divider. It takes the 5 V output from U1, and produces a voltage given by:

$$
V_{div} = \frac{R2}{R1 + R2} \times 5\ \textrm{V}
$$

Note that R2 is not a fixed 10 kΩ resistance, but rather a potentiometer (a variable resistance). As R2 varies from 0 to 10 kΩ, the voltage at the middle node (connected to C3 and the + input terminal of the op amp) varies from 0 to approximately 0.9 V.

OA1 is an op amp configured as a unity-gain buffer or voltage follower. Whichever the voltage applied at the non-inverting (+) input terminal of the op amp, the same voltage will be replicated at its output. While that seems pointless, it "shields" the resistive divider from voltage fluctuations arising from variations in the current drawn by the ECU. It's possible the buffer is not necessary, but you'd have to test. If you want a guaranteed result (unless the ECU draws an excessive amount of current, say tens of mA, which I find unlikely).

Regarding the op amp model: the LM321 should be easy to find. If not, look for an LM358. It has two op amps in a single package, but you're only going to use one (I suggest configuring the second one with the same circuit, but leave the output voltage terminal disconnected.) If you can't find either one, there are other models that will work, but you need to ensure they're 1. capable of working from a 14 V supply and 2. are of rail-to-rail type, at least with respect to the negative rail. The datasheets will give this information, but if you don't have experience with electronics, it may not be easy to find. You could add a comment here if you need further help.

Note that I'm assuming your sensor indeed behaves as a voltage source. If it is based on a different principle (current source, resistive, etc.) then this circuit won't work. Some extra details on the actual sensor being used (possibly an O2 sensor?) would help determine if the circuit is adequate.