WBO2 is an open-source, wideband O2 controller for use with Bosch LSU 4.x sensors, based on the Bosch CJ125 chip and an ATTiny1614 microcontroller.
I designed this to complement my ECU for my Triumph Spitfire restoration. Using a Bosch wideband oxygen sensor, it measures the air to fuel ratio of exhaust gases and outputs a proportional analogue signal. This data can be used by an ECU to determine what adjustments are required to the fuel injection quantity. The main goals of this project were to keep the component cost relatively low and to have a small form factor that could be sealed within the wiring loom.
It has diagnostics capability through different LED sequences and is completely open-source. You can see more details on the wiki.