Boston's Leading Mobile Mechanics

Avoid unnecessary repair  and  parts  costs.   The  Driveway  Doctors  have  the  latest in  computerized  diagnostic equipment and experienced ASE certified technicians to properly take care of your vehicle.

 

Computer Diagnostics and Analysis:

Performing a computerized car diagnostic test can reveal a number of problems associated with the transmission, exhaust system and other components of a vehicle.   Vehicles designed with computer processors, microchips and sensors can be linked to a car diagnostic computer scan to pinpoint exactly where the problem is.

 

Diagnostic trouble codes can range from P0100 to P1899.   Each letter and number in the code represents a different part of the vehicle.   The first letter is often a "P," which relates to the powertrain.   If it is a "B," it will relate to the body, a "C" relates to the chassis and a "U" is undefined.   The number zero in the second placeholder is a generic code shared by all manufacturers, but some use a specific code here so the number 1 may appear.   The third placeholder determines the area of the problem.   A 1 means there is a fuel or air problem;  2 means there is something wrong with the injector circuit;  3 means there is an ignition problem or the engine is misfiring;  4 means there is an emission control problem;  5 means there is a vehicle speed or idle control problem;  6 means there is a computer or output circuit problem;  a 7 or an 8 means there is a transmission problem.   The fourth and fifth digits of the computer diagnostic check report further identify which system is malfunctioning; the digits that appear here vary by vehicle make.

 

The Most Common Trouble Codes

There are common trouble codes that cause the Check Engine light to come on. These can make the vehicle fail an OBD II plug-in emissions test.   The following is a list of 20 most common trouble codes, with the percentage of failures for vehicles tested:

P0420 - Catalyst System Low Efficiency -                                                            13.2%

P0171 - Fuel Trim System Lean Bank 1 -                                                             10.4%

P0401 - Exhaust Gas Recirculation (EGR) Flow Insufficient -                              8.4%

P0174 - Fuel Trim System Lean Bank 2 -                                                               6.8%

P0442 - Evaporative Emission (EVAP) System Small Leak Detected -               6.7%

P0300 - Engine Misfire Detected (random misfire) -                                           6.4%

P0455 - Evaporative Emission (EVAP) System Leak Detected (large) -             6.2%

P0440 - Evaporative Emission (EVAP) System -                                                    5.5%

P0141 - Oxygen Sensor Heater (H02S) Performance Bank 1 Sensor 2 -          5.1%

P0430 - Catalyst System Low Efficiency Bank 2 -                                                 3.2%

P0135 - Oxygen Sensor (HO2S) Performance Bank 1 Sensor 1 -                      3.2%

P0446 - EVAP Vent Solenoid Valve Control System -                                           3.1%

P0128- Coolant Thermostat -                                                                                  3.1%

P0301 - Cylinder 1 Misfire Detected -                                                                    3.1%

P0411 - EVAP System Control Incorrect Purge Flow -                                         2.8%

P0133 - Oxygen Sensor Slow Response Bank 1 Sensor 1 -                                2.8%

P0303 - Cylinder 3 Misfire Detected -                                                                    2.6%

P0304 - Cylinder 4 Misfire Detected -                                                                    2.6%

P0302 - Cylinder 2 Misfire Detected -                                                                    2.6%

P0325 - PCM Knock Sensor Circuit -                                                                       2.1%

 

It would seem that the most common reason for the Check Engine light coming on and failing an emissions test would be a worn-out or fouled catalytic converter.  However, when you combine all related codes by system or component, you’ll get a different picture.  These are the systems that can cause an emissions failure:

 

Evaporative Emission System -                                                        24.3%

Engine Misfire -                                                                                   17.3%

Fuel Trim (lean) -                                                                                 17.2%

Catalytic converter -                                                                           16.4%

Oxygen sensor related -                                                                    11.1%

Exhaust Gas Recirculation EGR system -                                           8.4%

 

Causes of Common Trouble Codes

To pass an OBD II plug-in emissions test, there must be no trouble codes found. Simply erasing the codes with a scan tool won't fix the problem. The cause of the codes has to be diagnosed and repaired.

 

Causes of EVAP Related Codes P0411 P0440 P0442 P0446P0455:

The Evaporative Emission Control system prevents fuel vapors from escaping the fuel tank.   The EVAP system includes vent hoses and a charcoal canister for capturing and storing fuel vapors, and a valve for siphoning the fumes into the engine when it is running.  It also has a pressure or vacuum sensor for detecting small and large vapor leaks.   The most common cause of a P0455 EVAP Leak Code is a loose or missing gas cap.   A small leak code (P0442) may indicate a cracked or loose fuel tank vapor hose, a leaky EVAP storage canister, or a fault in the purge valve or EVAP leak detection system.   Check the gas cap to make sure it fits tightly.   The other problems can be very difficult to diagnose and they require professional help.   Technicians use a special "smoke" machine that lightly pressurizes the EVAP system and fuel tank to find leaks. The machine heats mineral oil to create a vapor-like smoke, which also contain UV leak detection dye that makes small leaks easier to find.   A professional level scan tool with bidirectional communication ability is necessary to cycle the purge solenoid and run other EVAP self-tests.

 

Causes of Engine Misfire Trouble Codes P0300 P0301 P0302 P0303 P0304 P0305:

Misfire codes don't tell why the engine is misfiring.    It only tells that one or more cylinders are not running properly.   The OBD II system tracks misfires by detecting changes in the speed of the crankshaft via the crankshaft position sensor while the engine is running.   A misfire causes a slight loss of speed in the rotating crankshaft, which the OBD II system logs as a misfire. A few misfires are normal, but if the engine experiences an excessive number of misfires within a given period of time, it will set one or more misfire codes. The last digit in the code indicates the number of the cylinder that is misfiring.   A P0300 code means the engine has a random misfire that jumps around from cylinder to cylinder.   P0300 random misfire codes are caused by vacuum leaks (loose or cracked vacuum hoses, leaky intake manifold gaskets, or a leaky vacuum brake booster), as well as a lean fuel mixture.   A lean mixture, which may also set a P0171 or P0174 code, means the engine is not getting enough fuel, or is getting too much air, possibly through a vacuum leak or a leaky EGR valve.   Dirty fuel injectors or low fuel pressure can be factors that cause a P0300 random misfire code to set. Bad gas that contains too much alcohol or water may cause this type of code to set. Cylinder specific misfire codes (such as P0301, P0302, etc.) tell a specific cylinder is misfiring, but this code does not tell why.   The cause might be ignition-related (worn or fouled spark plug, bad plug wire or coil-on-plug ignition coil), fuel-related (dead or dirty fuel injector), or compression-related (bent or burned valve or leaky head gasket).   All of these possibilities must be investigated to rule out the cause of the misfire.

 

Causes of Fuel Trim Trouble Codes P0171 P0174 A P0171 or P0174:

The trouble code tells you the fuel mixture is running lean (not enough fuel and/or too much air).   This problem can be confirmed by looking at the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values with a scan tool. Normally, STFT and LTFT should be plus or minus 5 to 10 from zero.   If STFT and/or LTFT are more than about +12, it indicates the engine is running lean. Negative numbers (-12 or more) indicates the engine is running rich.  A lean fuel condition can be caused by:

* Low fuel pressure due to a weak pump or leaky fuel pressure regulator.  Use a fuel pressure gauge to check fuel pressure at idle. If fuel pressure is less than specifications, the fuel filter may be plugged, the fuel pump may be failing or have a bad wiring connection, or the fuel pressure regulator may be leaking.

* Dirty fuel injectors.  Cleaning the injectors with a fuel system additive, or having the injectors professionally cleaned may solve the problem.

* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body.

* Leaky EGR valve.   Check the operation of EGR valve and system, and for a buildup of carbon under the valve.

* Leaky PCV Valve or hose.   Check the valve and hose connections. Dirty or defective Mass Airflow Sensor (MAF). Clean the MAF sensor wires or filament with aerosol electronics cleaner.  Do NOT use anything else to clean the sensor, and do not touch the sensor wires.

 

Causes of Catalytic Converter Codes P0420 P0430:

There is a "downstream" oxygen sensor to monitor the efficiency of the catalytic converter. If the converter is contaminated because the engine is burning oil or leaking coolant internally, or it is worn out from age, it will cause an increase in tailpipe emissions. The downstream oxygen sensor monitors the activity of the converter.   The engine computer compares the readings of the upstream and downstream oxygen sensors to determine how efficiently the converter is working.  If efficiency drops below a certain point, it sets a P0420 or P0430 code. Nine times out of ten, the code means the converter has reached the end of the road and needs to be replaced.There is no way to rejuvenate a failing converter, so replacing it is the only repair option. Removing it altogether is NOT an option as this is considered to be emissions tampering. A missing converter will cause the vehicle to fail the emissions test.

 

Causes of Oxygen Sensor Trouble Codes P0133 P0135 P0141:

Oxygen sensors monitor the amount of oxygen in the exhaust so the engine computer (PCM) can adjust the fuel mixture to minimize emissions and maximize fuel economy.   There are two types of oxygen sensor trouble codes: O2-heater circuit codes and O2 sensor codes.

A heater code will set if a fault is detected in the circuit that warms up the oxygen sensor when the engine is first started. This is necessary to reduce cold start emissions.

An O2 sensor performance code will be set if the O2 sensor readings remain low (lean), or high (rich), or do not change quickly enough, or do not change at all.   A low voltage (lean) oxygen sensor reading indicates the sensor has failed; or that it is being "fooled" by an exhaust manifold vacuum leak; a condition that allows unburned oxygen to enter the exhaust such as a burned or bend exhaust valve; or a misfiring spark plug. If you get an oxygen sensor code plus a random misfire code and/or a MAP sensor code, the engine has a serious vacuum leak.

 

Causes of EGR Trouble Codes P0401:

The Exhaust Gas Recirculation (EGR) system uses a vacuum or electronic valve between the intake and exhaust manifolds to recirculate the exhaust back into the intake manifold.  This occurs when the engine is at normal temperature and running under a heavy load.  The exhaust gas dilutes the air/fuel mixture slightly to reduce combustion temperatures.  This does two things; it reduces the formation of oxide or nitrogen (NOX) in the combustion chamber, and it helps the engine resist detonation (spark knock).  If the EGR valve fails, it can set an EGR trouble code.  The fix is to clean or remove carbon deposits under the valve or in the intake manifold EGR passages.

 

Causes of Coolant Thermostat Trouble Codes P0128:

The thermostat speeds engine warm up after a cold start and regulates the engine's operating temperature when it is running.   If the thermostat sticks shut, it can cause the engine to overheat.   If the thermostat fails to close, the engine never achieves normal operating temperature.   This prevents it from going into closed loop feedback control of the fuel mixture which makes the engine run rich and waste fuel.   This code indicates that the thermostat is not working properly.

 

Causes of Knock Sensor Trouble Codes P0325:

The knock sensor generates a signal when it detects engine vibrations that are typically produced by detonation (spark knock) during hard acceleration or when lugging the engine.   The PCM uses this information to retard spark timing slightly until the detonation stops.  The P0325 code may be set if the PCM gets a steady knock signal from the knock sensor.  The problem may be a faulty sensor or it may be operating conditions that are causing prolonged detonation.   The vibrations that may occur when driving on an unusually rough road may set a false knock sensor code.   Other conditions that may contribute to spark knock include engine overheating; low octane fuel; a buildup of carbon in the combustion chambers which raises compression; or loss of EGR.