The Air/Fuel Ratio Monitor has 20 LED's. Four red that indicate lean, Ten yellow that indicate stoichiometric ratio, and six that indicate rich. One LED lights up for every 50mV of oxygen sensor output.
Air / Fuel Ratio Meter and Oxygen Sensor Operation
The A/F Ratio Meter is a voltmeter with a range of 0 to 1 Volt. The meter displays the output voltage of the vehicles oxygen sensor through 20 LED's. The first LED will come on at a voltage of .050V, the second at .100V, the third at .150V, etc (see LED Chart on previous page).
The stoichiometric (STOICH) air/fuel ratio is the chemically correct ratio. This means theoretically all of the oxygen and all of the fuel are consumed. The mixture is neither rich nor lean. However, due to the fact that combustion is never perfect in the real world, there will always be a small amount of oxygen left in the exhaust. This small amount that is left is what the oxygen sensor measures. The smaller the amount of oxygen that is left in the exhaust, the richer the A/F ratio is, and the higher the oxygen sensor voltage is. The on-board computer or Powertrain Control Module (PCM) monitors the voltage from the oxygen sensor. If the PCM sees an oxygen sensor voltage greater than .450V, it immediately starts to reduce the amount of fuel that is metered into the engine by reducing the on time to the fuel injectors. When this happens, the A/F ratio starts to go in the lean direction, and the oxygen sensor voltage starts to go down. When the voltage drops below .450V, the PCM immediately starts to increase the fuel metered to the engine by increasing the on time to the fuel injectors to produce a richer A/F ratio. This occurs until the oxygen sensor voltage goes above .450V. This repeating cycle happens very fast (many times per second). The PCM is said to be in closed loop. It is constantly monitoring the oxygen sensor voltage and adjusting the on time of the fuel injectors to maintain a STOICH A/F ratio. This A/F ratio produces the lowest harmful exhaust emissions, and allows the catalytic converter to operate at peak efficiency, therefore reducing the exhaust emissions further.
Since the oxygen sensor output is very accurate and sensitive at the STOICH A/F ratio, it will cause the A/F meter LED's to bounce back and forth rapidly. A very small change in A/F ratio causes a large change in oxygen sensor voltage as can be seen on the graph. This rapid cycling is normal when the PCM is in closed loop and trying to maintain a STOICH A/F ratio. The oxygen sensor is also very accurate at indicating an A/F ratio that is richer or leaner than STOICH. However it can not indicate the exact richness or leanness due to the fact that the oxygen sensor output changes with the oxygen sensor temperature and wear. As the sensor temperature increases, the voltage output will decrease for a given A/F ratio in the rich area, and increase in the lean area as shown on the graph.
During wide open throttle (throttle opening greater than 80% as indicated by the throttle position sensor) the A/F ratio will be forced rich by the PCM for maximum power. During this time the oxygen sensor will output a rich signal/voltage, but the PCM will ignore the signal. The signal will be ignored because it is not accurate for indicating the exact richness. The PCM is now in open loop, and relies on factory programmed maps to calculate what the on time of the fuel injectors should be to provide a rich A/F ratio for maximum power. The A/F ratio meter should indicate rich during this time.
During hard deceleration the PCM will command an extremely lean mixture for lowest exhaust emissions. This may cause the signal to go outside the range that the meter will indicate. When this happens, there will be no LED’s lit on the A/F meter.