COBB Flex Fuel Sensor Kit – A Deeper Look

We first introduced COBB Subaru Flex Fuel Kits a little over 2 years ago.  These plug-and-play kits combined with custom tuning in the COBB Accesstuner software have made it easier than ever to transition between fuel types or ethanol blends on the fly in supported Subarus.  Gone are the days of nervously crunching numbers on your iPhone while standing awkwardly at the fuel pump.   This is all thanks to the work of some badass engineers that work here at COBB Tuning.


But how does it all work?

There are loads of terms such as sensors, converters, analyzers, and modules, so what’s what and how do they differ from each other?  Let’s take a deeper look at the hardware side of the COBB Flex Fuel offering.
First up is the fuel sensor itself.  Like many other solutions, our kit uses a sensor manufactured by Continental.  This is the exact sensor used by major manufacturers producing vehicles with flex-fuel capabilities. There are several “knockoff” parts available in the marketplace, however, we order sensors directly from Continental.

As fuel flows through this sensor, a digital pulse signal is generated to indicate ethanol content of the fuel passing through it.   The frequency of this pulse coincides with the ethanol content of the fuel passing through the sensor and updates about 4-5 times per second.  It should be noted that while the sensor can output very precise changes, down to 0.1%, the part that actually reads ethanol content is accurate to ~10% under normal operating temperatures.


These sensors detect ethanol content by capacitance – an electrical value that changes based on the relative permittivity or what we’ll call the “electrical signature” of the substance between the two layers.

Because of this, the sensing element must be fully immersed in fuel to read accurately.  During the initial investigation, this became readily apparent when different vehicles would have erratic ethanol content readings at high loads.  We found the cause of this to be one of two things.  Either the fuel pumps were causing fuel cavitation or the sensor itself was placed in an un-pressurized return.   When placed in this location, the senor would drain of fuel when the fuel pressure regulator was only releasing a small amount flow through the return. We have taken these lessons and incorporated them into our current flex-fuel solutions.

When the sensor detects an electrical signature close to that of gasoline, it outputs slower pulses. For the (higher) electrical signature of ethanol, it outputs faster pulses. It should be noted that when this sensor detects something that is not gasoline *or* ethanol, the pulses go FAR outside the normal range to signal an error. Because water – which is readily absorbed by ethanol – has a signature about double that of ethanol, it only takes a very small amount to throw the sensor into an error state.  We have also seen fuel additives (such as ethanol fuel conditioners) and fuels that contain something other than standard unleaded gasoline or ethanol, such as race gas, cause the fuel sensor to report an error.  The manner in which those signals are handled is what differentiates most aftermarket flex fuel kits.

This leads us to the Flex Fuel Module.


A component unique to the COBB Flex Fuel kits is the module, which takes digital pulses from the Continental fuel sensor and converts them to a signal usable by the car’s ECU.  Since Subaru ECUs cannot interpret the digital output directly from the fuel sensor, it must be converted to analog.  While other options on the market take the fuel sensor’s data and convert it to analog voltages, they differ based on how they output that analog voltage, how they handle error states, and how they’re physically manufactured and sealed.

Some kits will output signals within a 0-5V range, while others, like ours, output within a 0.5-4.5V range.  Luckily, the small difference in range is negligible in accurately measuring ethanol content so the smaller voltage output range doesn’t really matter. So why do it? Simple – things don’t always go according to plan.

With our 0.5-4.5V range, we can trigger a COBB-custom check engine light and put the ECU into a safe state when voltage goes out of range. Furthermore, our modules will output a voltage outside of the normal range depending on the specific sensor error.  This is vital when troubleshooting potential mechanical issues.  In our experience with other kits available, they try to send the last known good reading until the error code subsides.  This leaves the user completely in the dark when the sensor is no longer reading properly.  While we designed our software to be compatible with these 0-5V kits, the downside is that 0V will always mean 0% ethanol to your car, and 5V will always mean 100%.  The downside of using those kits is their inability to trigger a warning for wiring failure or short, equipment failure, or fuel contamination.

For additional information about COBB Custom DTCs related to Flex Fuel tuning, see the article linked here:

Flex Fuel Related COBB Custom DTCs

In short, a lot of thought and effort went into developing the plug-and-play COBB Flex Fuel Kits.  When paired with a properly calibrated custom tune, they are second to none.  Feel free to reach out to our Customer Service Team or comment below if you have questions!

Leave A Comment