What Does Catalyst Monitor Incomplete Mean?

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The catalytic monitor measures the ability of your vehicle’s catalyst converter to reduce harmful emissions. When working as intended, the oxygen from the exhaust gas is used to clean up pollutants and other harmful components in the exhaust. 

This process is monitored and regulated by the onboard diagnostic system (OBDII) which consists of monitors that track the vehicle’s performance. There are two types of monitors in modern vehicles: continuous and non-continuous.

While the former keep operating as long as the engine is turned on, the latter requires certain conditions to be met, like speed, acceleration, ambient, fuel level, and more, to be able to proceed with the testing process.

Catalyst monitor falls in the non-continuous category and analyses your catalytic converter’s ability to reduce the emission of harmful gases. Sometimes, the diagnostic system may indicate “Catalyst Monitor Incomplete” which indicates the testing hasn’t taken place.

Here’s what you need to know about this issue and how to fix it:


Catalyst Monitor – What Is It?

A catalyst monitor is a diagnostic tool that checks the efficiency of a car’s catalytic converter in reducing emissions. The catalytic converter is a component of the exhaust system that converts pollutants into less harmful substances. 

It compares the levels of exhaust gasses and oxygen in the exhaust to ensure the catalytic converter is working properly and meeting emissions standards. If the ratio of these gasses is not within the proper range, it may indicate that the catalytic converter is not functioning properly. 

In this case, the catalyst monitor will trigger a warning on the dashboard, usually in the form of a “Catalyst Monitor: Incomplete” message.


How Does a Catalyst Monitor Work?

The catalyst motor takes the Oxygen from the exhaust gas and uses it for the breakdown of pollutants. The monitor uses the PCM (powertrain control module) to measure the oxygen entering and leaving the catalytic converter under normal operating conditions. 

It compares the readings from upstream and downstream oxygen sensors to determine the converter’s efficiency. If the converter is functioning properly, there should be a small amount of unburned oxygen in the exhaust. 

The catalyst monitor also uses an upstream oxygen sensor during closed-loop fuel control to measure the oxygen content in the exhaust gas by detecting oxygen pulses produced by each cylinder’s combustion event. 

The sensor produces a voltage signal that oscillates in response to these oxygen pulses. The downstream oxygen sensor, which is located at the outlet of the catalytic converter, measures the oxygen content of the exhaust gas after it has passed through the catalytic converter.

If the downstream sensor reading is fluctuating in a similar way to the front sensor, it means that the catalytic converter is not functioning properly and may be allowing emissions to exceed the federal certification limit by 1.5 times.

If the catalytic converter is not functioning properly, the PCM (powertrain control module) will activate the check engine light to alert the driver. In order for the catalyst monitor to be activated by the PCM, the check engine light must be turned off and the continuous monitors, including the oxygen sensor monitor, must have been successfully completed. 

The PCM can initiate the catalyst monitor when the engine is operating normally at steady highway speeds. To heat the catalytic converter to the proper testing temperature of over 1000 degrees Fahrenheit, the PCM adds extra fuel to the converter through the fuel injectors.

It then compares the upstream and downstream oxygen sensor signals to evaluate the efficiency of the catalytic converter.


Reasons for Catalyst Monitor Incomplete Issue

Here’s a list of the various reasons why your catalyst monitor is showing as “Incomplete” on the dashboard: 

Disconnected Sensor/Cells

Disconnecting sensors and cells can erase the memory in the PCM, including fault codes and previous results from OBD monitors. This resets the system as if it were new. It may take a long time, possibly even weeks or months of highway driving, for all the monitors to be activated again by the PCM. 

If sensors are malfunctioning, they can prevent the monitors from functioning properly and may generate multiple error codes.

Stored Codes Removal

Resetting the stored codes can also reset all monitors to their initial state, requiring the sensors to start again and using up a lot of power. Some of the deleted fault codes may reappear, potentially causing problems for one or more sensors during their operation. 

This can lead to incomplete catalyst monitor issues and should be avoided if possible.

Fitting Performance Issues

Modifying the programming and engine control module can have significant effects on power, shift points, and the tachometer’s calibration for different gear ratios or tire sizes.

However, this type of modification can also negatively impact certain internal monitors by deactivating low-level oxygen sensors and preventing catalyst monitors from functioning properly.

It is important to carefully consider the potential consequences of these types of modifications before implementing them.

Deceiving The System

Performance enthusiasts who attempt to “cheat” emissions testing by using heavily modified vehicles are often the cause of catalyst monitor incomplete issues in Toyota vehicles.

These types of modifications can be tempting, but it is important to remember that they can have negative consequences and may not be legal in certain areas.

It is generally best to avoid using these types of tricks and to ensure that your vehicle is operating properly and within legal limits.


Common Signs of Incomplete Catalyst Monitor

Here’s a list of some common signs that indicate there’s something wrong with the catalyst monitor: 

Poor Acceleration/Loss of Engine Efficiency

One of the most noticeable signs of a malfunctioning catalytic converter is a loss of power during acceleration, especially on steep inclines. This is a common issue in Honda vehicles, and a simple test to diagnose the problem is to have a friend hold the turbo spools at around 1,800 to 2,000 rpm while you observe the exhaust airflow.

If the gases are hot, it may indicate a deficiency. When the catalytic converter becomes clogged and unable to effectively process exhaust gases, it can lead to increased pressure and strain on the engine. This can cause the vehicle to shake and stall due to the inability to properly vent combustion byproducts. This problem is also common in Jeep vehicles.

Rattling Sounds/Poor Exhaust System Capacity

If there are issues with the fuel system, it can result in a significant decrease in the performance of the exhaust. This can be caused by a faulty catalytic converter, as seen in 2003 GMC Envoy vehicles with incomplete catalyst monitor and EVAP monitor operations.

Another common symptom of a malfunctioning catalytic converter is a rattling sound during idle, which is often located at the back of the vehicle. This type of problem is prevalent in 2006 Mercedes ML350s with incomplete catalyst monitoring functions and may sound similar to the noise of a lawn mower.

Additionally, a minor shaking at start-up that becomes more intense over time may be a sign of a severely degraded honeycomb porcelain lattice inside the catalytic converter.

Bad Fuel Economy/Sulfur-like Odor

If you detect a strong, rotten egg-like odor coming from the trunk of your vehicle, it may be a sign that the catalytic converter is no longer effective at absorbing hydrogen sulfide, a sewage gas with a distinctive odor.

Another indicator of a malfunctioning catalytic converter is poor fuel efficiency, which can be easily noticed from the driver’s seat. If the highway MPG falls below 10% of its typical value, it is likely that the catalytic converter is not functioning properly.

It is also important to check the gasoline pump and fuel filter, as these components may be the root cause of the problem.

Check Engine Light Turned On

If there is a problem with the catalytic converter, the check engine light will turn on as a warning. Some newer vehicles have air-to-fuel ratio meters and oxygen detectors to monitor exhaust emissions, which can help identify problems with the catalytic converter. 

However, the check engine light can be triggered by various engine faults, not just issues with the catalyst monitor. To accurately diagnose the cause of the problem, it is recommended to have the vehicle inspected by a professional mechanic. 

A catalyst monitor bank 1 test incomplete inspection may be a good place to start.

Broken Fuel Vapors

If your car has a carburetor, you can perform a simple test to check if it is functioning properly. Here’s how:

  • Remove the air filter.
  • Ask a friend to hold a flashlight near the fuel tank while you look for a cloud of gasoline vapor (if the carburetor is clogged, there should be a visible vapor).

If you are able to identify a broken vapor, it is best to take your vehicle to a mechanic for a more comprehensive diagnostic report. The mechanic will assess the fuel level, the intensity of any vacuum leaks, and the engine temperature to determine whether a new carburetor is necessary.


How To Fix Incomplete Catalyst Monitor Issue

Here is a step-by-step process to help you get rid of the issue in no time:

  • Step 1: Check that the engine warning light is off. If the car has active fault codes, it can prevent the monitor from functioning at full capacity.
  • Step 2: Make sure that the coolant temperature is within the normal range. The engine coolant temperature should be below 50°C (122°F) and within six °C (11°F) of the ambient air temperature. One way to achieve this is to allow the car to sit overnight.
  • Step 3: Start the car and let it idle for about two minutes, or three and a half minutes, with the rear defroster and air conditioning turned on.
  • Step 4: Turn off the rear defroster and air conditioning and drive the car at a constant speed of 55 mph (90 km/h) for approximately three minutes. During this time, the purge and diagnostics for the fuel monitor will occur naturally.
  • Step 5: Gradually reduce the speed of the car to 20 mph (32 km/h) without using the brake or clutch.
  • Step 6: Accelerate the car back up to 55 mph (90 km/h) for about five minutes at 3/4 power, allowing the system to perform basic catalyst monitor tests. If the batteries have been disconnected or the catalysts are not ready, it may take five full driving cycles to prepare the car.
  • Step 7: Bring the car to a stop and allow it to idle for two minutes.

If you have followed these steps and are still experiencing catalyst monitor incomplete issues, it is recommended to visit a mechanic as soon as possible for further diagnosis and repair.


Final Words

In conclusion, a catalyst monitor’s incomplete status means that the vehicle’s catalytic converter is not functioning properly due to various factors, including faulty oxygen sensors, a clogged converter, or modifications to the vehicle’s programming or engine control module.

If you are experiencing any of these issues, it is important to have your car inspected by a professional mechanic to determine the root cause of the problem and determine the appropriate course of action.



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