DTC P0172 PDF

P is a trouble code that is set when the ECU senses that there may be too much gasoline present in the air-fuel mixture of the engine. A proper air-fuel ratio of about What the P code means P indicates that there is too much gasoline being detected in the exhaust gases exiting the combustion chamber. The ECU uses a number of instruments, such as the mass air flow sensor MAF , oxygen sensors, and manifold absolute pressure MAP to monitor the air-fuel ratio of the engine.

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Your engine needs to put in the right amount of fuel for the amount of air coming into it, that is, the proper air-fuel ratio, to deliver the best power and fuel economy.

As suggested by the names, the HO2S is sensitive to oxygen O2 content in the exhaust, which has a direct relationship to air-fuel ratio. On combustion, some of the oxygen in the air is used to oxidize the fuel, leaving a small amount in the exhaust, which the HO2S measures, sending a signal back to the ECU.

If there is a lot of oxygen, this means that the engine is running lean, that is, not enough fuel was injected. Conversely, if there is very little oxygen, this means that the engine is running rich, that too much fuel was injected. On the other hand, engine problems, such as high fuel pressure, a skewed MAF signal, or a faulty fuel injector, may dump in more fuel than required, or misreport how much air is entering, resulting in a rich condition.

What are the common causes of code P? Depending on year, make, and model, DTC P may have number of causes. Here are some of the most common. Discard the old MAF and engine air filter. Clean the air box and install the new MAF and air filter, preferably using OEM parts, making sure that the air box seals properly. Oiled Filters — Some engines with a lot of blow-by, whether by design or age, and may allow oil into the air box, and subsequently, the air filter and MAF. Also, some aftermarket air filters require oil treatment for proper operation, which some owners may be overzealous in applying.

In either case, excessive oil can contaminate the MAF, leading to skewed air flow measurements. Also, check that the PCV and tubing are in proper working order.

Dirt — In an ideal world, your engine bay would be spotless. Over time, however, dirt and grime builds up, and it can easily find its way into the engine and control systems. Older and neglected vehicles can easily suffer MAF contamination, for excessive dust and dirt or a poorly-sealed air box.

Leaking Fuel Injector — Age or contamination may cause a fuel injector to leak, dumping fuel into the cylinder even when not commanded. Check the oil for fuel smell, which is a good indicator of fuel leakage. Leaking Fuel Pressure Regulator — Some vehicles, with vacuum-operated fuel pressure regulators, have been known to leak internally, passing unmetered fuel directly into the intake.

What are the symptoms of code P? Depending on the nature of the failure, DTC P may or may not be accompanied by drivability issues. On the opposite end of the scale, you may note rough idling, lack of power on acceleration, even misfiring or stalling, which tends to get progressively worse as the engine warms up.

Particularly bad cases may be accompanied by black exhaust smoke and a blinking MIL, alerting you to possible catalytic converter damage. You may also smell a strong fuel odor and see black soot in the exhaust tip or the back of the car, an indicator that a lot of unburnt fuel is making it through the system.

How do you troubleshoot code P? Pro Tip: Before diagnosing fuel trim problems, it does one well to note that the Fuel Trim Monitor will not run unless the HO2S monitors have run and passed first. The fuel trim monitor uses the oxygen sensor to run, which means that fuel trim codes are not oxygen sensor codes.

Instead, focus on the fuel injection system and the MAF. MAF contamination could skew air intake measurements, therefore skewing fuel injection calculations. Cleaning may solve the problem. Check fuel pressure, making sure that it is within the proper range. Excess pressure, such as caused by a defective pressure regulator or pinched return line, would result in more fuel being injected than the ECU is expecting.

Fuel Pressure Drop — With the engine running, record fuel pressure, then shut the engine off. Fuel pressure may drop slightly, but should remain stable for at least 10 or 15 minutes. If the fuel pressure continues to drop, you could have a leaking fuel injector, which would lead to a rich condition and possible misfire condition. Cylinder Misfire — A misfiring cylinder would dump unburnt fuel into the exhaust stream. Diagnose and repair a cylinder misfire before attempting to diagnose a rich condition.

Exhaust — Check the exhaust for leaks between the cylinder head and oxygen sensor. While the exhaust system seems like a positive-pressure system, air can enter in the partial vacuum created by the exhaust pressure pulses. Atmospheric oxygen entering before the HO2S would skew oxygen content measurements. Even though I have been working on cars for a long, long time, this one is making me pull my hair out. Honda Odyssey Idles rough initially but smooths out Can anyone point me in the right While driving, the engine will occasionally stumble a bit.

This has been going on for the last several of years,

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