Car Talk

Incomplete Catalyst Monitor – Signs, Causes, and Fixes

Avatar Adie Tinkle -

Incomplete catalyst monitor have always been among the most widespread problems with automatic cars and most drivers do not know how to fix them properly. Therefore, the owners need to have proper guidelines to deal with this issue. Before getting assistance from a professional, check out some of the tips in this blog to assess what solutions you can implement independently. 

What is a Catalyst Monitor?

The catalyst monitor assesses the converter’s ability to reduce air pollution. When functioning correctly, catalysts use the oxygen from the engine exhaust to decompose other gas constituents.

A properly functioning catalyst uses oxygen from the exhaust stream and breaks down other exhaust gas components.

In a typical closed-loop fuel control system, all the oxygen takes entry into the converter. The Powertrain Control Module (PCM) constantly tracks the oxygen levels entering and leaving the converter and estimates the converter’s efficiency.

Very little unburned oxygen is left when it exits an active converter. 

How Does Catalytic Monitor Work?

The upstream oxygen sensor receives a flash of oxygen through each cylinder’s combustion event when closed-loop fuel management is enabled.

The oxygen sensors produce an oscillating voltage output in response to oxygen pulses. It depicts that all the available oxygen has been consumed by the converter. 

You can identify the malfunction with a number of fluctuations between high and low. When oxygen sensor signals show a loss of converter efficiency, the PCM turns on the check engine light.

Before enabling the catalytic converter monitor, the PCM must turn off the check engine light and finish the continuous oxygen sensor monitoring. When the engine turns at steady-state highway speeds, the PCM can initiate the monitoring system. 

By pumping extra fuel into catalytic converters, the PCM can elevate its internal temperature by 1000 Fahrenheit. The PCM compares the upstream and downstream oxygen sensor signals. 

If the catalytic converter’s efficiency is greater than 50%, the PCM will finish the catalytic converter monitoring process successfully. 

A pending code will be set by PCM if it determines that the converter’s efficiency is less than half of its maximum. Failing in the first two drive cycles will cause the PCM to turn on the check engine light and store a diagnostic trouble code. 

Signs of Incomplete Catalyst Monitors

If you find the following signs in your vehicle, take it right away to the professionals. 

  1. Poor acceleration and engine efficiency loss 
  2. Rattling sound because of poor exhaust system capacity 
  3. Check engine light sign 
  4. Broken fuel vapors 
  5. Bad fuel economy and sulfur-like odors

Let’s discuss these points in detail.

1. Poor Acceleration and Engine Efficiency Loss 

Accelerating power failure is one of the most significant signs of malfunctioning converters, particularly on steep trips. A simple test for that is getting a hole in the turbo spools at around 1,800 to 2,000 RPM when you are handling exhaust airflow.

When the catalyst gets clogged and is not able to handle exhaust gases, the engine will face challenges leading to a flush of combustion byproducts. 

It can cause suffocation of the gas tank and horsepower engine, causing the vehicle to tremble and stall as a result of unexpected pressure surges. 

2. Rattling sound because of poor exhaust system capacity 

The exhaust’s performance can experience a drastic drop because of complications with the fuel system that glitches directly from a broken converter. 

A rattling sound when your vehicle is idle is also an indication that can be observed from the back of your vehicle. When starting the vehicle, you might also hear slight shaking that only increases as time passes. The cause behind this can be severe deterioration and breakdown of the honeycomb porcelain lattice inside the catalytic converter. 

3. Bad fuel economy and sulfur-like odors

One of the most pungent gases absorbed by a catalytic converter is a sewage gas with a distinctive rotten egg-like odor. 

If you are detecting foul stench emanating from the trunk of your car, there is a high chance that the system has lost its efficiency to an intolerable degree. Moreover, poor fuel efficiency is a solid indicator that you can easily detect from the driver’s seat. If the vehicle’s MPG falls below 10% of its regular value, the chances are that the catalytic converter is also malfunctioning. 

4. Check the Engine Light On 

Your check engine light can turn on at the slightest signal of broken converters. Its warning light will let you know that your system is not performing optimally. The newer version also includes an air-to-fuel ratio meter and oxygen detectors to inspect exhaust emissions. Even with all that, multiple engine faults can be the possible cause behind this trigger.

The Catalyst Monitor is Not Ready. But Why?

There are tons of possibilities as to why the catalyst monitor is incomplete, including a broken rear defroster, numerous exhaust leaks, malfunctioning ignition switch, and other similar issues. 

The four primary causes include 

  1. Disconnected sensors and cells 
  2. Removal of stored codes through a scan tool 
  3. Fitting performance tunes through an automotive spare
  4. Deceiving the system through simulator installation  

1. Disconnected Sensors and Cells 

The catalyst monitor’s incomplete chevy suffers from these issues. The disconnected cells and sensors remove all the PCM’s memory, including the fault codes, before OBD can monitor the recorded test results. It seems like everything has been reset to zero. It can take up to weeks and even months of highway driving for all the monitors to activate the PCM again. 

Also, the non-function sensors prevent the monitors from operating properly and most likely generate several error codes. 

2. Removal of Stored Codes through a Scan Tool 

Another error that can lead to the incomplete catalyst monitor issue is the removal of stored codes through a scan tool.  It also sets all monitors to zero and drives sensors to reoperate, causing a waste of tons of power supplies. 

As the removed fault codes return, at least one of the sensors can deal with glitches during the procedure. Some immature drivers even extract the catalytic converter from their car, which not only hampers the monitor to an irreversible extent but is also illegal. 

3. Fitting performance tunes through an automotive spare 

Doing this programming results in changing the engine control module entirely. The modifications are also supposed to boost power, calibrate the tachometer for altering gear ratios, and alter shift points. 

A major downside to this strategy is their destructive infliction on some of the internal monitors. They deactivate low-level oxygen sensors and prevent the catalyst monitor from operating.

4. Deceiving the System through Simulator Installation

These common tricks are usually performed by enthusiasts who try to pass emission catalytic testing with heavily modified automobiles. 

Catalyst incomplete problem is the prime consequence of this scheme. It also leads to serious lawful penalties and the doer will also be disqualified and forced to remove all the statistical changes inflicted on the vehicle. 

How to Fix Catalyst Monitor? 

Even though there is no single solution to all the issues, some of these steps might come out to be effective strategies for managing them. 

  1. Make sure that the engine warning light is off 
  2. Ensure that the coolant temperature remains normal 
  3. Let the engine run idle for 2 minutes 
  4. Operate the vehicle at a constant 55 mph 
  5. Reduce your vehicle’s speed by 20 miles per hour 


Now you have a complete idea about the primary facets of catalyst monitor. The catalyst monitor determines if the catalytic converter is functioning properly. All monitors, including the catalyst monitor, rely on sensors to work efficiently. There are various reasons behind an incomplete catalyst monitor. If the fixes discussed above do not work out to resolve the problem, it is best to consult a professional.

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