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Failed Emission Test Codes: Most Common Trouble Codes

Based on Illinois Environmental Protection Agency data test, the most common trouble codes that cause an OBD II plug-in emissions test failed on many vehicles and make the Check Engine light on would be fouled or a worn out catalytic converter. More common trouble codes that cause vehicles fail on emissions test and make the Check Engine light coming on can found below.

DescriptionPercentageTrouble Code
Coolant Thermostat3.1%P0128
Oxygen (O2) Sensor Slow Response Bank 1, Sensor 12.8%P0133
Oxygen Sensor (HO2S) Performance Bank 1, Sensor 13.2%P0135
H02S (Oxygen Sensor Heater) Performance Sensor 2, Bank 15.1%P0141
Fuel Trim System Lean Bank One10.4%P0171
Fuel Trim System Lean Bank Two6.8%P0174
Engine Misfire Detected (random Misfire)6.4%P0300
Cylinder 1 Misfire Detected3.1%P0301
Cylinder 2 Misfire Detected2.6%P0302
Cylinder 3 Misfire Detected2.6%P0303
Cylinder 4 Misfire Detected2.6%P0304
PCM Knock Sensor Circuit2.1%P0325
EGR (Exhaust Gas Re-circulation) Flow Insufficient8.4%P0401
EVAP (Evaporative Emission) System Control Incorrect Purge Flow2.8%P0411
Catalyst System Low Efficiency13.2%P0420
Catalyst System Low Efficiency Bank 23.2%P0430
Evaporative Emission (EVAP) System5.5%P0440
Evaporative Emission (EVAP) System Small Leak Detected6.7%P0442
EVAP Vent Solenoid Valve Control System3.1%P0446
EVAP (Evaporative Emission) System Leak Detected (large)6.2%P0455

Even fouled or the worn out catalytic converter is the most cause failed on test, based on above data you will get a somewhat different picture, if all of the related codes by system or component to be combined. There’s a systems that are most likely cause fail on emissions test.

Problem DescriptionPercentage
Evaporative Emission System24.3%
Engine Misfire17.3%
Fuel Trim (lean)17.2%
Catalytic converter16.4%
Oxygen sensor related11.1%
Exhaust Gas Re-circulation (EGR) system8.4%

Causes of Common Trouble Codes

Self-tests on all of the OBD system monitors must have run and completed to pass an OBD II plug-in emissions test, and there’s should no trouble codes found during the test. If one or more trouble codes appears on your vehicles test, erasing the the codes won’t fix the issue, you need to diagnose the troubles code(s) and repair the cause of the trouble codes appear. Make sure there no new trouble codes appears again, you should OBD monitors self-test run completely (you can verify it using a scan tool). This the only way to make your vehicles pass on the OBD II plug-in emissions test.

P0300 P0301 P0302 P0303 P0304 P0305 Engine Misfire Trouble Codes and The Causes

If your engines is misfiring the misfires code won’t tell you why it’s misfiring, the trouble codes only show you cylinders that don’t run properly. Detecting subtle changes in the crankshaft position sensor when the engine is run, it’s how the OBD II system tracks the misfires. Speed in the rotating crankshaft will slight loss caused by misfire, and the OBD II system logs will say as a misfire. It’s normal when you have a few misfires, however the misfires codes will be sets one or more, if in given period of times the engine experiences an excessive number of misfires. The last two digits of misfire codes will tell you the cylinders number that misfiring. If you have P0300 misfires code, it’s mean misfires jumps around around from cylinder to cylinder and it a random misfires on engine.

P0304 - Cylinder 4 Misfire
P0304 – Cylinder 4 Misfire

Lean fuel mixture, leaky vacuum brake booster, leaky intake manifold gaskets, cracked or loose vacuum hoses, or another vacuum leaks is the cause of misfires code P0300 appear. The P0171 or P0174 misfire code is set usually caused by lean mixture, it can means the engine is getting too much air, or engine is not getting enough fuel, possibly through a leaky EGR valve or a vacuum leak. Some factors that cause random misfires will set code P0300 such as low fuel pressure or dirty fuel injectors. This type code also can be caused by bad gas that contains too much air or alcohol.

Misfire codes such as P0301, P0302, etc. is cylinder specific misfire codes, where the last two digits the code is refer to misfiring cylinder, it just let you know the cylinder that misfire and will not tell you why the cylinder is misfiring. Possible causes can vary such as compression-related (leaky head gasket or burned or bent valve), fuel related (dirty or dead fuel injector), ignition-related (coil-on-plug ignition coil or bad plug wire, fouled or worn spark plug). If you want to get rid of the cause of misfire, all these possibilities should be investigated.

P0411, P0440, P0442, P0446, P0455: (EVAP Related) ANd The Causes

Fuel vapors will not escaping from the fuel tank due to the Evaporative Emission Control. To storing or capturing fuel vapors, the EVAP system includes a charcoal canister and vent hoses, and the siphoning the fumes will be sucked into the engine (when it running) by the purge valve. Also, it has vacuum sensor or a pressure sensor for detect vapor leaks (small and large). Missing or losing a gas cap is the common cause the EVAP Leak Code P0455 appear. EVAP leak detection system or a fault in the purge valve, a leaky EVAP storage canister, loose or cracked fuel tank vapor hose will be indicated by a small leak code (P0442). For gas cap problem, make sure it fits tightly, and you can check it yourself. However to diagnose another problem, it will be difficult and usually you need a professional help. To find leaks, the technicians doing a lightly pressurizes the fuel tank and the EVAP system using a special “smoke” machine. Small leaks easier to find, because to create a vapor-like smoke the machine heats the mineral oil, it may contain dye of UV leak detection. Cycle the purge solenoid and other EVAP self-test can be run using a professional level scan tool which have a bidirectional communication ability.

P0411 - EVAP System Control Incorrect Purge Flow
P0411 – EVAP System Control Incorrect Purge Flow

Cause of lean fuel condition

1. Low Fuel Pressure
Leaky fuel pressure regulator or weak pump can cause low fuel pressure. When engine idle check the fuel pressure using fuel pressure gauge. If the check result tell fuel pressure less than specifications, it could be the fuel pressure regulator may be leaking, you have a bad wiring connection or your fuel pump may be failing, or your fuel filter may be plugged.

2. Dirty fuel injectors
To fix the problem, use fuel system additive to clean the injectors or clean the injector professionally by a technician.

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

4. EGR Valve Leak
Check EGR system and valve, and carbon under the valve buildup.

5. PCV Valve or hose leaks.
Check hose connections and valve connections.

6. Defective or dirty MAF (Mass Airflow Sensor)
Using aerosol electronics cleaner, clean the sensor wires or filament of Mass Airflow Sensor. Do not touch the sensor wires and do not clean the sensor with anything else.

P0133, P0135, P0141: Oxygen Sensor Trouble Codes And The Cause

Oxygen amount in the exhaust monitoring by oxygen sensor, so the fuel mixture can be adjusted by the PCM in order to maximize fuel economy and minimize emissions. O2 sensor performance codes and O2 heater circuit codes are two types of oxygen sensor trouble codes. Circuit that warms up the oxygen sensor when your engine is first started, will be detected as fault by Oxygen Sensor, and it will set a heater codes. This is needed to decrease cold start emissions. If the O2 sensor readings remain high (rich), or low (lean), or don’t quickly enough to change, or don’t changed at all, it will set an O2 sensor performance code.

You may have a misfiring spark plug, or a condition that allows unburned oxygen to enter the exhaust such as a burned or bend exhaust valve, or exhaust manifold vacuum leak “fooling” the sensor or the sensor has failed, all of it indication a low voltage (lean) oxygen sensor reading.

NOTE: There are a serious vacuum leak problem on the engine, when you get a random misfire code and/or a MAP sensor code plus an oxygen sensor code.

P0420, P0430 Catalytic Converter Codes And The Causes

To monitor the efficiency of a catalytic converter, the OBD II vehicles is monitored by a “downstream” oxygen sensor. Increasing in tailpipe emissions due the converter does not work because it has become contaminated. Contamination occurs because it is worn out from age, or leaking coolant internally or engine burning oil. Converter activity is monitored by the downstream oxygen sensor. The efficiency of the converter can be determined by reading upstream and downstream oxygen sensors, and to determine the level of efficiency, then the computer will compare the results. Code P0420 or P0430 will be set if the efficiency drops below a certain point.

The converter has reached the end of the road and needs to be replaced if the output code is nine times out of ten. The only way to fix failing converter is to replace it, because the failed converter can not be rejuvenated. It will be considered an emission tampering, if you remove it altogether, and that is not an option. Your vehicle will fail on the emissions test, because converter is missing.

P0420 - Catalyst System Low Efficiency
P0420 – Catalyst System Low Efficiency

Causes of P0401 EGR Trouble Codes

To recirculates a small amount of exhaust back into the intake manifold, the EGR (he Exhaust Gas Re-circulation) system uses a electronic valve or vacuum actuated between the intake and exhaust manifolds. This happen when the engine running under a heavy load or engine is accelerating and engine at a normal temperature. To reduce the combustion temperature, the exhaust gas dilutes the air/fuel mixture slightly. There are two things; it helps resist detonation of the engine (spark knock) and the fuel chamber of oxide or nitrogen (NOX) formation is reduced. EGR trouble code will be set if the EGR valve fails, or it isn’t flowing properly due to an accumulation of carbon under the valve. The way to fix is EGR valve and system need to be observes and tested, and carbon deposit in the intake manifold EGR passages or under the valve need to clean up or removed. Defective DPFE sensor in the EGR system causing EGR codes are often occurs on Ford vehicles.

P0171, P0174 Fuel Trim Trouble Codes And The Causes

Misfire codes P0171 or P0174 indicated the engine too much air and/or engine is not enough fuel (fuel mixture running lean). Using a scan tool you can looking at the values the STFT (Short Term Fuel Trim) and LTFT (Long Term Fuel Trim) to confirm the problem. STFT and LTFT normal values is 5 to 10 (plus or minus) from zero. The engine is running lean when you have value more than +12 for STFT and/or LTFT. And the engine is running rich when you get negative values for minus 12 or more.

P0325 Knock Sensor Trouble Codes And The Causes

During hard acceleration or when lugging the engine, detonation (spark knock) typically will produce an engine vibrations and will make the knock sensor generates a signal. Before he detonation stops, the PCM will use this information to retard spark timing slightly. If the knock sensor send a steady knock signal to PSM, may be it will set the P0325 code. The cause of prolonged detonation could be an operating conditions or faulty sensor problem. False knock sensor code will be set if the vibrations occur on an unusually rough road wile you are driving. Loss of EGR, low octane fuel, engine overheating or raises compression due to a carbon buildup in the combustion chambers may contribute to spark knock problem.

P0128 Coolant Thermostat Trouble Codes And The Causes

The thermostat will speeds the engine warm up right after a cold start, also regulates the temperature of engine’s operating when the engine is running. Overheat will occur if the thermostat sticks shut. he engine may never achieve normal operating temperature if the thermostat fails to close. It could preventing the fuel mixture going into closed loop feedback control, that may causing waste fuel and the engine run rich. The thermostat is not working properly if this code set the conditions.

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