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Open and Closed Loop Modes:
Open or closed loop operation refers to the way the computer is deciding how much
fuel to add to the air entering the engine. During cold start and other low temperature
situations, the computer operates in open loop mode. This means that it is relying on
a set of internal calculations and data tables to decide how much fuel to add to the
incoming air. It uses sensors such as the coolant temperature sensor (CTS), the
throttle position sensor (TPS), and the manifold absolute pressure sensor (MAP) to
determine optimum mixtures. The important difference here is that it does not check
to see if the mixtures are correct, leaving the computer adjustment loop open.
In closed loop mode, the computer still decides how much fuel to add by using the
sensors listed above, and by looking up the appropriate numbers on a data table.
However, it now checks itself to determine whether the fuel mixture is correct. It is
able to check itself by using the information provided by the oxygen sensor(s) (O2S)
in the exhaust manifold. The oxygen sensors will tell the computer if the engine is
running rich or lean, and the computer can take steps to correct the situation. In this
way, the computer closes the adjustment loop by checking itself and making neces-
sary corrections. It should be noted that the O2 sensors must be at a very high
operating temperature (approximately 650° F) before they will begin to feed informa-
tion back to the computer. This is why open loop mode is necessaryto give the O2
sensors time to warm up to operating temperature.
As long as the engine and O2 and Coolant Temperature Sensors are at operating
temperature, the computer can operate in the closed loop mode. Closed loop mode
constantly corrects to obtain an air/fuel mixture at the ideal 14.7:1. But in stop and go
cycles, the O2 sensor may in fact cool down enough that the computer will need to
rely on a set of internal parameters and go into open loop mode again. In some
cases, this may also happen during extended periods of idling. Many newer vehicles
now use heated O2 (HO2S) sensors to prevent this condition.
In many vehicles, the computer controls other systems related to open and closed
loop modes, including idle speed, electronic spark control, exhaust gas recirculation,
and transmission torque converter clutches. In open loop mode, some of these
systems will be adjusted to speed the warming of the engine and get the computer
into closed loop mode as quickly as possible.
OBD II
In 1994, manufacturers began equipping vehicles with a new class of computer
technology which puts more processing power under your dash than ever before. It is
called On-Board Diagnostics, Second Generation, or OBD II. It is required on all
vehicles sold in the US beginning January 1, 1996 (though most domestic manufac-
turers introduced it earlier than required), and offers increased system monitoring
and diagnostic information. This new system stores a library of more than 650
general trouble codes and another approximately 400 manufacturer-specific codes,
all of which you can access with the OBD II System Tester. These codes cover Body
Systems (B-Codes), Chassis Systems (C-Codes), Communications Codes (U-
Codes), and Power Train Systems (P-Codes). Now, basic terms are standardized
and all generic codes will share a common format and terminology that the manufac-
turers and the Society of Automotive Engineers (SAE) designed. You will be glad to
know that as your vehicle gets smarter, it will be easier for you to keep track of what is
going on under the hood.