Navigating the complexities of your car’s On-Board Diagnostics II (OBDII) system can be daunting, especially when preparing for a smog test. A common hurdle many car owners face is understanding OBDII codes and ensuring their vehicle is ready for emissions testing. If you’ve encountered an OBDII code related to a failed engine temperature sensor, or are struggling with “incomplete” monitors after a battery disconnect or clearing codes, this guide is for you. We’ll break down how to address these issues and utilize the OBDII drive cycle to get your car ready for its smog check.
Understanding OBDII Monitors and Readiness
Modern vehicles are equipped with sophisticated OBDII systems that continuously monitor various engine and emission control components. These monitors ensure your car is running efficiently and within environmental regulations. For a vehicle to pass a smog test, most of these monitors need to be in a “ready” state. However, certain events like battery replacement or clearing diagnostic trouble codes (DTCs) can reset these monitors to an “incomplete” status.
Think of OBDII monitors as system checks. They evaluate components like:
- Catalyst: Checks the efficiency of the catalytic converter in reducing emissions.
- EVAP System (Evaporative System): Monitors for fuel vapor leaks in the fuel system.
- O2 Sensor (Oxygen Sensor): Evaluates the performance of oxygen sensors critical for fuel mixture control.
- EGR (Exhaust Gas Recirculation): Checks the EGR system’s function in reducing NOx emissions.
- And more, depending on your vehicle’s make and model.
When these monitors are “incomplete,” it simply means the OBDII system hasn’t yet run all the necessary tests to confirm these systems are functioning correctly since the last reset. In many regions, including California, vehicles are allowed a limited number of incomplete monitors to still pass a smog test. However, having too many incomplete monitors will lead to failure.
The OBDII Drive Cycle: Your Solution for Smog Test Readiness
The OBDII drive cycle is a specific series of driving conditions designed to allow your vehicle to run all its onboard diagnostic checks. By completing a drive cycle, you give your car the opportunity to test each monitor and set them to a “ready” status. While “just driving it” might eventually set the monitors, a drive cycle is a targeted and faster approach.
It’s crucial to start the drive cycle with a “cold start.” This means the engine coolant temperature should be below 122 degrees Fahrenheit (50 degrees Celsius) and close to the ambient air temperature (within 11 degrees Fahrenheit or 6 degrees Celsius). Avoid turning the ignition key to the “on” position before starting the engine, as this can interfere with certain monitor tests, particularly the heated oxygen sensor diagnostic. Also, ensure your fuel tank is between 1/4 and 3/4 full.
Step-by-Step OBDII Drive Cycle Instructions
Here’s a general OBDII drive cycle based on common recommendations. Always consult your vehicle’s specific service manual for the most accurate drive cycle procedure, as it can vary by manufacturer and model year.
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Cold Start and Idle: Start the engine and let it idle in drive (or neutral for manual transmissions) for two and a half minutes. Turn on the air conditioning and rear defrost to increase engine load. This step checks the oxygen sensor heater circuits, air pump, and EVAP purge system.
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Accelerate to 55 mph: Turn off the A/C and rear defrost. Accelerate smoothly to 55 mph (88 km/h) at half throttle. This engages tests for ignition misfire, fuel trim, and canister purge.
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Maintain 55 mph Cruise: Hold a steady speed of 55 mph for three minutes. During this phase, the OBDII system monitors the EGR, air pump, O2 sensors, and canister purge systems.
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Decelerate to 20 mph: Coast down to 20 mph (32 km/h) without braking or depressing the clutch. This step checks EGR and purge functions during deceleration.
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Accelerate to 55-60 mph: Accelerate again to 55 to 60 mph (88-96 km/h) at ¾ throttle. This re-engages tests for misfire, fuel trim, and purge.
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Maintain 55-60 mph Cruise (Extended): Hold a steady speed of 55 to 60 mph for five minutes. This extended cruise allows the OBDII system to thoroughly monitor catalytic converter efficiency, misfire, EGR, fuel trim, oxygen sensors, and purge functions.
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Decelerate to Stop: Coast down to a complete stop without braking. This final deceleration allows for a last check of the EGR and canister purge systems.
After performing steps 1-4, you can check your OBDII monitor status using a scan tool like the Autel AL619 or similar. You may find that some monitors have already switched to “ready.”
What if the Drive Cycle Doesn’t Work? Addressing OBDII Codes
If you perform the drive cycle and your monitors remain incomplete, or if you are facing a specific OBDII code, further investigation is needed. An OBDII code for a failed engine temperature sensor, for example, such as P0118 (Engine Coolant Temperature Circuit High Input) or P0119 (Engine Coolant Temperature Circuit Intermittent), can prevent the drive cycle from completing successfully.
A faulty engine temperature sensor can provide incorrect readings to the engine control module (ECM). This can disrupt various engine operations, including fuel injection and ignition timing, and critically, it can hinder the OBDII system’s ability to run its diagnostic tests correctly. If the ECM detects a problem with the engine temperature sensor, it may not initiate or complete certain monitors, leaving them in an “incomplete” state and potentially triggering a “Check Engine Light.”
In such cases:
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Check for OBDII Codes: Use an OBDII scanner to read any stored trouble codes. Address any fault codes, especially those related to the engine temperature sensor or other emission control components, before attempting the drive cycle again.
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Repair Faults: If you find a code for a failed engine temperature sensor, replace the sensor. Similarly, address any other identified issues.
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Clear Codes and Retry Drive Cycle: After repairs, clear the OBDII codes with your scan tool and then repeat the OBDII drive cycle.
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Professional Diagnosis: If the monitors still don’t set to “ready” after repeated drive cycles and addressing any fault codes, it’s advisable to seek professional diagnosis from a qualified mechanic. There might be underlying issues preventing monitor completion that require specialized tools and expertise to identify.
Passing a smog test requires your vehicle’s OBDII system to confirm that its emission control systems are functioning correctly. Understanding the OBDII drive cycle and addressing potential issues like a failed engine temperature sensor are key steps to achieving smog test readiness and keeping your vehicle running efficiently.
