We need to know exactly which emission(s) category your Toyota Corolla failed in. Did your Toyota Corolla fail the smog test because the CO, HC or NOx emissions were high? Was the failure in only one of these emission categories or a combination of more than one? The answer will determine which part of the emissions system is responsible for the overall failure.
It would be best to have a smog check repair center conduct a Fuel Feedback Test to get a general idea of how well your Toyota's engine is controlling fuel delivery. A feedback test will tell if your Toyota Corolla is in proper fuel control, and if air/fuel ratio is as close to 14.7:1 as possible or not. 14.7:1 air/fuel ratio is needed in order to burn fuel thoroughly and produce low emissions. During the feedback test, a smog technician will check all emission components which are responsible for sensing air intake, oxygen levels in the exhaust, and fuel delivery, as well as whether your Corolla's ECU (engine control unit) is receiving and responding to sensor data properly.
A common smog check failure for older model vehicles is high HC emissions. The most typical faults in this category have to do with improper spark delivery; caused by a defective or damaged ignition system components. We are referring to your Toyota Corolla's spark ignition system. Not to be confused with the key ignition system.
While diagnosing high HC faults it is important to know how much CO the vehicle produced during the smog check as well. A byproduct of incomplete combustion is CO (partially burned fuel). High CO along with high HC indicates a rich mixture problem, while low CO and high HC indicates a misfire condition due to an inadequate amount of fuel entering the combustion chamber. In both scenarios, whether high or low CO was produced, a byproduct of the inefficient combustion will be high HC.
Some Common HC Faults Include:
Vacuum Leaks - Ensure your Toyota Corolla is not suffering from any vacuum leaks. During idle, intake manifold vacuum is at its highest. Any vacuum leak (after the throttle plate) will cause a lean mixture and increase HC emissions. Once the engine is off idle, HC will return to normal because the fuel injection system will present more fuel to the combustion chambers, neutralizing the vacuum leak. So, checking for vacuum leaks is usually first step.
Ignition and Spark - Once a quick visual inspection has been conducted for disconnected, brittle, or broken vacuum hoses, move on to inspecting the ignition system wiring. Spark plugs and spark plug wires should not show wear, be brittle or cracked. The spark plugs should not appear fouled (heavy carbon build-up) or ashed (whitish appears due to high heat). The distributor cap and rotor must also be inspected for defect or damage.
MAP (Manifold Absolute Pressure) Sensor - The MAP sensor should be checked for proper operation as well. The MAP (manifold absolute pressure) sensor is responsible for letting your Toyota Corolla's ECU know how much intake vacuum exists. This simple and inexpensive sensor is crucial for proper air/fuel mixture ratio at both idle and off-idle conditions.
How to Test Toyota Corolla MAP Sensor - You may test the MAP sensor using a voltmeter set to measure voltage. You should see an increase in voltage as RPM increase (intake vacuum decreases). At idle you should see between to 1 to 1.5 volts. Off idle, voltage should start increasing to a maximum of 5.0 volts.
When looking the MAP sensor you will notice it has three prongs which a connector connects too. The MAP sensor signal measurement should be taken from the center prong (middle wire of the three wire connector). You must leave the MAP sensor connected during this test, and back probe the connector. If you have a hand vacuum pump, you may use it to apply vacuum to the sensor, as opposed to running the engine. Disconnect the vacuum line leading from the intake manifold to the sensor and attach your hand vacuum pumps vacuum line instead. Turn your Toyota's ignition on, do not start the engine, and conduct the MAP sensor test. If no voltage exists, either at no vacuum or 20in HG vacuum, inspect the other two wires for power (5 volts) and ground. One wire should have constant 5 volts and the other have good ground.
