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Interpreting OBD2 DataThe generic OBD2/ODBII data is used extensively in modern engine control. The information below attempts to demonstrate some of the more common uses for the data. Calculated Engine Load California Air Resource Board (CARB) defines this as airflow divided by peak air flow. Peak air flow may be adjusted for altitude via barometric means if available. For diesel engines, the number is current output torque divided by peak output torque at current rpm. We have noted that vacuum leaks can have dramatic effects on this value. Coolant Temperature Modern engine performance depends heavily on the accuracy of the sensor monitoring coolant temperature. This sensor is often independent of the measurement reported on the dash. It should read close to ambient temperature at startup, and run to between 180°F-220°F after warmup. Injector pulse width is heavily dependent on this sensor, causing engines to run very rich at startup (remember the manual choke?). Fuel enrichment is usually tapered off around 150°F. Fuel Trim Fuel trim refers to adjustments being made dynamically to the base fuel table to get the proper ratio of fuel to air. Short term fuel trim refers to adjustments being made in response to temporary conditions. Long term fuel trim is used to compensate for issues that seem to be present over a much longer period. Fuel trims are expressed in percentages; positive values indicate lean (add fuel) and negative values indicate rich (subtract fuel). Fuel trim banks refer to the cylinder banks in a V style engine. Cylinder #1 is always in bank 1. Fuel trim is generally calculated by using a wide set of data values, including front O2 sensors, intake air temperature/pressure (or the more elegent air mass sensor), engine (coolant) temp, anti-knock sensors, engine load, throttle position (and change in throttle position), and even battery voltage can effect fuel trim. Long term fuel trim generally should not exceed +- 10%. Manifold Pressure Manifold Air (or Absolute) Pressure (MAP) sensor measures intake vacuum, used to calculate engine load and timing advance. It may also contribute to fuel trim. O2 Sensors O2 sensors monitor exhaust gases for the presence of oxygen. Heated sensors are commonly referred to as HO2S, which allows sensors to begin giving valid data much earlier in the warm up cycle. Most sensors output a voltage between 0 and 1.0 volts (usually .10 to .90), 0v - .45v for lean conditions, and .55v - 1.00v for rich conditions. Upstream sensors are primarily used to adjust engine air and fuel flow, while downstream (of the catalytic converter) sensors are used to monitor the effect of the catalytic converter. Sensors do not usually run at constant voltages, rather oscillate rapidly (at least once every few seconds) in a nice sine wave. The frequency is referred to as switching frequency, or as switches. The rear sensor should display a much slower switching frequency than the front sensor, the difference indicating the catalyst capability. These sensors are invaluable in the diagnostic effort, and can indicate conditions ranging from fuel delivery issues, vacuum leaks, to engine oil overfill conditions. Ford has an Ho2s Service tips page with lots of generally useful info, and some great Ford specific stuff. 
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