LambdaLink Air/Fuel Ratio Meter :: Subaru Impreza RB5 Owners Club
As well as detonation, another potential problem is a bad air/fuel mixture within the engine. Like every other car manufactured since 1980, the Impreza uses an Oxygen Sensor (aka "O2 Sensor" or "Lambda" probe) to monitor the air/fuel mixture. The goal of the O2 sensor is to help the engine run as efficiently as possible and lower exhaust emissions.
A petrol engine burns petrol in the presence of oxygen and it turns out that there is a particular ratio of air and fuel that is "perfect". This is known as the "stoichiometric" ratio and is is 14.7 parts of air to 1 part fuel (14.7:1)
If there is less air than this perfect ratio, then there will be fuel left over after combustion. This is called a rich mixture. Rich mixtures are bad because the unburned fuel creates pollution. The Impreza runs rich by default as this helps maintain the life of the engine parts.
If there is more air than this perfect ratio, then there is excess oxygen. This is called a lean mixture. A lean mixture tends to produce more nitrogen-oxide pollutants, and, in some cases, it can cause poor performance and even engine damage. Running too lean is not good for your engine and will shorten is life expectancy.
On the 99/00 Impreza's, the oxygen sensor is positioned in the Downpipe (in the Manifold on earlier cars) and can detect rich and lean mixtures. The mechanism involves a chemical reaction that generates a voltage. The engine's computer looks at the voltage to determine if the mixture is rich or lean, and adjusts the amount of fuel entering the engine accordingly.
There's plenty of Air/Fuel Ration monitors on the market, but after noticing a Group Buy on Scoobynet, I decided to get a Link Systems LambdaLink. The monitor, checks the voltage of the Oxygen Sensor and displays this on a group of LED's. The monitor can tell you if the O2 sensor is working properly and if the car is running dangerously lean or overly rich under load.
If the oxygen sensor fails, the ECU can no longer sense the air/fuel ratio, so it ends up guessing. When my O2 sensor failed in the middle of 2001, the symptoms included poor idling (revs would bounce between just a hundred to 1,000 rpm), poor fuel consumption and high exhaust emissions.
As well as detonation, another potential problem is a bad air/fuel mixture within the engine. Like every other car manufactured since 1980, the Impreza uses an Oxygen Sensor (aka "O2 Sensor" or "Lambda" probe) to monitor the air/fuel mixture. The goal of the O2 sensor is to help the engine run as efficiently as possible and lower exhaust emissions. 
