Great question. Turning off the check engine light requires resetting the engine control computer (ECU). After an ECU reset, the check engine light turns off and trouble codes are erased but at the same time the reset erases all the emission monitors. The "monitors" are internal tests which the ECU performs to check itself and specific systems related to engine emissions, i.e. the EGR system, the EVAP system, Catalyst, so on.
In this particular case, a trouble code was registered by the ECU indicated a problem with the engine temperature sensor (ECT). There may be a problem with the ECT, or perhaps a fluke may have caused the trouble code to be set, i.e. weather condition confusing the ECU into thinking the engine warmed up too quickly or not quickly enough. This is rare but it could happen. For that reason, your smog technician choose to simply reset the ECU and see whether or not the trouble code returns.
We'll assume the trouble code has not returned and the check engine light has not illuminated. This is not to say that the ECT does in fact not have a fault. The ECT is a sensor which measures engine coolant temperature and accordingly increases or decreases electricity flow through its terminals. The sensor may be worn but not to the point of effectively triggering the check engine light on. With that said, it should be noted that because the ECT is not defective "enough" to trigger the CEL on it does not mean that it is operational enough to allow any one monitor which relies on ECT data to run/initiate. In other words, the ECT may appear to be functioning because it has not yet triggered a trouble code, however it may not be functioning well enough to allow the catalyst monitor to run.
Our recommendation is to have your smog technicians diagnose the ECT system. Using a voltmeter they will be able to measure the accuracy of this sensor and determine whether it needs replacement.
The catalyst monitor not being ready is not an indication of a bad catalytic converter. There are various parameters (sensor data) which must be met (including data from the ECT) before the catalyst monitor will initiate, and after successfully passing internal self-tests, show as Complete or Ready.
Since your Honda Civic's monitors were erased due to the reset, you'll need to complete a Honda Civic Drive Cycle in order to set the emission monitors.
Note: Performing the drive cycle will only set the required monitors if there aren't any emission related problems your car is experiencing. We mention this because, if after at least 2 attempts, the drive cycle does not allow all emission monitors to complete, your Honda Civic might have an emissions problem which will need repair first; in this case, perhaps the ECT.
Usually driving in both city and highway conditions over the course of a few days is enough to get the drive cycle to run and set the monitors. At times however, due to individual driving habits, following the recommend drive cycle is required.
Follow the drive cycle carefully. Take extra caution when performing the drive cycle on public roadways. If you feel you will be unable to follow the drive cycle safely, please seek assistance from an smog check repair station or Honda service center. They may be able to perform the drive cycle utilizing a dynamometer, and specialized scan tools. Perform the drive cycle at your own risk.
How To Perform a Honda Civic Drive Cycle
Cold Start: Begin from Cold Start, and ignition in OFF position for at least 1 hour. ensure your Honda’s fuel tank is between 1/4 and 3/4 full and engine coolant temperature must be below 122°F and within 11°F of the ambient air temperature. Do not leave the key on prior to the cold start or the heated oxygen sensor diagnostic may not run.
Idle Time: Run your ’s engine (do not drive) for 2 1/2 minutes with the air conditioner on and rear defroster on. During this period the ECU will be running the Oxygen Sensor Heater self-test, Air Injection System Passive Air test, EVAP Purge "No Flow" test, Misfire Detection.
Acceleration: Turn off the air conditioner and rear defroster and begin driving. As soon as possible apply half throttle until 55mph is reached. The ECU will now run self-tests on the Misfire Detection, Fuel Trim, and EVAP Purge Flow.
Hold Steady: Now hold your vehicle speed steady at 55mph for three minutes. During this time the Oxygen Sensor O2 response, Air Intrusive (AIS System), EGR Flow, Purge Valve, Misfire Detection, and Fuel Trim diagnostics will be performed.
Deceleration: During this step you want to slowly let off the accelerator pedal. Do not brake or shift your Honda in order to slow down. It is important to let the vehicle coast along gradually slowing down to 20 mph. The ECU will once again self-test the EGR System, EVAP Purge System and Long Term Fuel Trims.
Accelerate: Go ahead and accelerate again at 3/4 throttle until 55-60mph is reached. This will perform the same self tests as during the first acceleration step.
Hold Steady: Hold steady speed once again. This time at 55mph for five minutes. During this time, in addition to running the self-tests as during the first HOLD STEADY step, the ECU will perform the catalyst monitor diagnostics. If the catalyst (Catalytic Converter) is marginal or the battery has been disconnected, it may take 5 complete driving cycles to determine the state of the catalyst,or in order to Complete the CAT Monitor.
Decelerate: This will perform the same diagnostics as in the first Deceleration. Remember not to press your Honda’s brake or shift gears during deceleration.
To ensure all monitors are complete, use a scan tool to view the monitors.
