The Throttle Position Sensor can get out of adjustment due to wear in the throttle body or due to carbon build up. This will cause the idle timing to advance more than 30 degrees which will cause very high HC and NOx emissions. Conversely, the Throttle Position Sensors can wear out and not properly advance the timing which causes a lack of power and poor fuel economy.
1997 Toyota Corolla Problem Reports
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The rear Oxygen sensors are a critical element in fuel control (unlike other vehicles) and can cause an emissions test failure for high CO and HCs. The sensing element cracks and and reports an erroneous steady state lean condition which confuses the engine computer.
The idle air control motors tend to become carboned up at around 100,000 miles and can cause a start and stall and/or stalling at stop sign problems. Our technicians tell us that many times these motors can be cleaned, however, the sure bet is to replace the idle air control motor and clean the throttle body.
The timing belt tensioner may make a rattling noise at a higher mileages. This is due to the adjustment being at its limit. Our technicians tell us the range of the adjuster may be lengthened by elongating the slot where the spring attaches by 2-3 mm to remedy this problem.