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The oxygen sensor measures the amount of oxygen in the exhaust, telling the computer whether there is too much or too little fuel being injected. In four cylinder cars, there are usually two sensors — one up front, and a second sensor that checks on the first and on the efficiency of the catalytic converter. In cars with V-type engines, there can be two banks of sensors, one for each bank of cylinders. There are separate codes for each sensor - bank 1 and bank 2, sensor 1 (forward) and sensor 2 (aft).
In some cases, oxygen sensor codes are thrown when there really is a problem somewhere else. Sometimes they are thrown when there is an issue with the wiring or grounding.
Some cars have two banks of oxygen sensors. The oxygen sensor is heated at startup to get it functioning properly as quickly as possible — hence the "heater control circuit" errors.
“Slow response” indicates a “lazy” sensor which should be replaced. “Heater performance” errors involve the built in electric sensor heater which gets it into operating mode more quickly.
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