In the Last diagnostics article, we outlined how to retrieve and decipher engine diagnostics for engines running OBD-I engine management computers. OBD or On Board Diagnostics is a subsystem of the car's ECU that can monitor the engine's emissions system.

When there's a failure in emissions components or if the engine runs in an inefficient manner (misfires, air-fuel ratio etc.) the check-engine lights up and the computer records the error code in its memory. In our last Diagnostics article, we highlighted the code reading tools and other techniques needed to get codes from domestic and imported vehicles, touched on the basics of retrieving codes and outlined the meanings of those codes.

Here, we're picking up in 1996 when the emission system computer was updated and standardized. This meant all the different methods of retrieving codes were standardized, right down to the diagnostic port plug that connects the computer to a code reader. The plug is located inside the cabin, either under the center dash, left dash or behind the ashtray.

However, it's the software side of OBD where the big changes went down in 1996. Basically, the OBD system was changed from reading whether or not the emission system was functioning to how well it was functioning. OBD-I was looking for a yes or no answer. The problem is, emission systems do not fail all at once, they tend to deteriorate over time or run at, say, 25 percent efficiency. Second-generation OBD systems are precise enough to sniff out these shortfallings. OBD-II monitors more sensors and systems, is capable of detecting more errors and even saves a snapshot of engine operating parameters at the time of an error.

Monitoring
Generally speaking, there are 11 different OBD-II monitors, which can be divided into two groups; continuous and single. Continuous monitors are made up of a CCM (Comprehensive Component Monitor), Misfire Monitor and Fuel System Monitor. The single-type monitors perform their assigned task once per driving cycle. These monitors consist of Oxygen Sensor Monitor, Oxygen Sensor Heater Monitor, Catalyst Monitor, Heated Catalyst Heater Monitor, EGR Monitor, EVAP Monitor, Secondary Air System Monitor, Air Conditioning Monitor.

OBD-II Type Casting
An OBD-II trouble code comes in two varieties. Type 1 codes trigger the check engine light immediately after a failure is detected. If the check engine light is flashing, that means there's dangerous misfire that can damage the catalytic converter. The proper code is saved in the computer's memory and a snapshot of the engine's operating parameters is taken. These parameters include fuel system status, engine load, water temperature, fuel ratio, MAP vacuum and engine speed.

A Type 2 code is detected but the check engine light is not immediately illuminated. This "pending" code is saved in the ECU's memory. If the code is fired during the next drive cycle, the ECU trips the check engine light and takes an engine parameter snapshot. If there is no repeat in error, the pending code is erased.

Once the check engine light is tripped, it'll stay illuminated until the conditions that triggered the light are no longer present for three consecutive driving cycles. If this occurs, the light will go off but the code will remain in the memory for 40 cycles.

Code Grabbing
To illustrate OBD-II code grabbing, we are using an Equus 3100 code reader that can be ordered direct from J.C. Whitney. The code reader is plugged into the diagnostic port, the key is switched on and the Link button is pressed. If the check engine light is on, the unit will flash a five-digit code, which reveals where the trouble code originated. If there's more than one code in the memory, the scroll button is used to move through the codes.