Common Reasons for Failure

1975-1995 Gasoline Vehicles
1996-Current Gasoline Vehicles
1975-2001 Diesel Vehicles
2002 and Newer Diesel Vehicles


What Are the Most Common Causes of Failures for 1975- 1995 gasoline vehicles?

These are general guidelines only.

High HC Emissions

  • Inoperative/missing catalytic converter
  • O2 sensor malfunction
  • Internal engine problem
  • Vacuum leaks (hoses, vacuum operated devices, intake manifold, carburetor)
  • Ignition system malfunction (plugs, plug wires, points, dwell, etc.)
  • Improper fuel injector operation
  • Incorrect ignition timing and/or idle speed
  • Incorrect carburetor air/fuel mixture
  • Defective EGR valve or air injection system failure

High CO Emissions

  • Excessively rich air/fuel mixture
  • O2 sensor malfunction
  • Faulty computer control(s)
  • Malfunctioning fuel injection system
  • Inoperative/missing catalytic converter
  • Carburetor float level maladjusted
  • Air cleaner, choke or carburetor
  • Defective canister purge system


Possible Causes of High Hydrocarbon (HC) Emissions

Hydrocarbons relate to unburned fuel. You may logically think that must mean that the engine is getting too much fuel. However, that is only one of many possibilities ranging from fuel problems, to electrical problems, to internal engine problems such as piston rings that can cause HC emissions to be excessive. In order to pinpoint the cause of excessive HC emissions, the following systems (if applicable) will need to be checked, usually in the order shown below:

Lean or Rich Air-Fuel Ratio

For an engine to operate as designed, the correct ratio of fuel to air must be delivered to the cylinders. If the fuel system is delivering a leaner than ideal air-fuel ratio, it may result in lean misfire and cause high hydrocarbons. If the fuel system is too rich, it also may result in high HC but will be accompanied by high CO as well.

Inadequate Catalytic Converter Efficiency

For vehicles that are 1988 model year or newer, it is very important that the catalytic converter be operating at 90% efficiency or better. That means that the emissions that come out of the tailpipe must be no more than 10% of what goes in.

Induction System Problems

There are many aspects of the engine’s air induction system (hoses, intake runners, intake manifold, vacuum-controlled devices) that can cause disruption in the air and fuel getting to the cylinders and result in high HC. Incorrect PCV valve/orifice flow rate can also cause similar symptoms. Leaking EGR valves also may cause excessive HC emissions.

Poor Ignition Performance

Ignition defects including dirty spark plugs, leaking or open-circuited spark plugs or wires, or defective ignition coils can all result in a shortage of spark energy. Any shortage of spark energy may cause high HC emissions.

If the spark occurs at the wrong time, incomplete combustion and high hydrocarbons may result. Too much spark advance could be due to an incorrect adjustment or a defect in mechanical or vacuum advance mechanisms.

Uneven Output Among the Engine’s Cylinders

At this stage of the diagnosis the technician should be sure that the air-fuel ratio is correct, there are no external vacuum leaks, and the ignition system is operating normally. There are still many possible causes of high hydrocarbons, most of which are either internal engine problems or tough-to-detect induction system problems such as valve deposits or a leaking EGR valve. On fuel-injected vehicles, poor fuel atomization is a common cause of HC problems.


For all of these remaining possibilities, checking for uneven power output amongst cylinders will usually help to track down the cause of the problem.

Possible Causes of High Carbon Monoxide (CO) Emissions

High CO means too much fuel. Fuel can only come from three sources: the crankcase vapor control system, the evaporative control system, or the actual fuel delivery system.

The fuel delivery system is certainly the most likely culprit but in most cases it is very quick and easy to eliminate the other two possibilities first. However, if it is problematic to check the evaporative system purging, it is probably wise to go straight to fuel delivery system diagnosis.

In order to pinpoint the cause of excessive CO emissions, the following systems and possible defects will need to be checked, usually (but not always) in this order:

Excessive Crankcase Blowby or PCV Flow

If oil vapors in the engine’s crankcase are excessive, or the oil is dirty or contaminated, excessive CO emissions can result. Dirty or contaminated oil is easily rectified by an oil change. Excess crankcase vapors flow can be caused by an incorrect PCV valve or by serious internal engine damage such as worn-out piston rings. Serious internal engine damage can only be remedied by major engine repair or overhaul.

Saturated Evaporative Control System

The charcoal canister stores fuel system vapors until they can be withdrawn and burned in the engine. Under normal circumstances, this should never result in excessive CO for more than a few seconds. It is possible for charcoal canisters to become saturated with fuel. This essentially means that they can never be cleared of HC vapors and need to be replaced.

Rich Air-Fuel Mixture

There are many aspects of the engine’s fuel delivery system that may need to be checked when diagnosing a rich air/fuel mixture. That is the main reason that this is listed behind crankcase vapor and evaporative emission control systems-even though they are less likely causes of the problem, they are much easier and quicker to check.

When diagnosing a rich air-fuel mixture, it is important to keep in mind the possibility of a defect in an entirely different area affecting the operation of the fuel delivery system. For example, certain mechanical engine defects may cause abnormally low engine vacuum and result in the fuel system delivering more fuel than is actually needed by the engine. The symptom of excess CO may lead one to think that the defect lies with the fuel system but the problem actually lies elsewhere.

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What Are the Most Common Causes of Failures for 1996 – Present gasoline vehicles?

On-Board Diagnostic Failure (1996 and newer light-duty vehicles only)

The cause(s) of an OBD failure is dependent upon the diagnostic trouble code(s) stored in the vehicle’s on-board computer. The most common causes are:

  • Air/fuel mixture out of control range
  • Catalytic converter efficiency too low
  • EGR system malfunction
  • Misfire
  • Evaporative control system malfunctionction

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What Are the Most Common Causes of Failures for 1975- 2001 Diesel vehicles?

High Opacity (Diesels Only)

  • Injection timing/pump timing maladjusted
  • Incorrect fuel pump pressure
  • Injectors fouled, leaking or mismatched
  • Restricted air intake system
  • Dirty air cleaner
  • Engine mechanical defect

Possible Causes of High Diesel Opacity Emissions

In order to pinpoint the cause of excessive diesel opacity (smoke), the following systems (if applicable) and possible defects will need to be checked, usually in this order:

Restricted Airflow

Any restriction to the flow of air into the engine can cause excessive smoke emissions. This includes the air filter, air inlet pipes or ducts, and the intake manifold. Similar to the air filter and inlet ducts, the turbo pressure also can affect the smoke emissions of a diesel engine. The technician will need to ensure that the turbo is running at the correct speed and pressure. A restricted exhaust system may also cause a reduction in the airflow into an engine under certain circumstances.

Injection Timing

The technician must make sure that the injection pump is timed to the correct cylinder and that timing is set to manufacturer’s specifications.

Injection Quality

Diesel fuel injectors that are restricted can cause excessive smoke. Other injector defects such as worn valve seat, sticking nozzle, and incorrect opening pressure may also cause excessive smoke. To test or clean diesel injectors, they will need to be removed and bench tested according to the manufacturer’s recommended procedures and specifications.

Exhaust Gas Recirculation Devices

Exhaust Gas Recirculation (EGR) devices are used to control NOx emissions in many diesel vehicles but if the rate of EGR flow is excessive, smoke emissions can increase significantly.

Oxidation Catalyst

The performance of the catalyst can have a significant effect on smoke emissions. The technician should refer to the manufacturer’s recommended procedures and specifications for testing of these components.


Engine compression is a measure of piston/cylinder sealing integrity and should be checked with a compression test as per manufacturer’s recommended procedure. Compression pressures for each cylinder should be within 10% of each other.

Injection Pump Calibration

If all else fails, the injection pump may require an overhaul or re-calibration.
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What Are the Most Common Causes of Failures for 2002 – Present Diesel Vehicles?

On-Board Diagnostic Failure

The cause(s) of an OBD failure is dependent upon the diagnostic trouble code(s) stored in the vehicle’s on-board computer. The most common causes are:

  • Air/fuel mixture out of control range
  • Catalytic converter efficiency too low
  • EGR system malfunction
  • Misfire
  • Evaporative control system malfunction

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