P0172 – System too rich, bank 1

By Benjamin Jerew (Contact Me)
Last Updated 2015-12-17
ASE Master Tech

Trouble CodeFault LocationProbable Cause
P0172 System too rich, bank 1 Intake blocked, EVAP canister purge valve, fuel pressure, EGR system, injector(s), HO2S

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What Does Code P0172 Mean?

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. The ECU (engine control unit) uses a number of sensors, such as the MAF, CKP, and ECT (mass air flow, crankshaft position, and engine coolant temperature) to determine how much fuel to put in. Using feedback from the HO2S, AFR (heated oxygen sensor or air-fuel-ratio sensor), or Lambda sensor, the ECU can fine-tune fuel injector pulse-width (IPW) to deliver the best air-fuel ratio.


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. Under normal operation, the ECU cycles between rich and lean air-fuel ratios, at the same time learning short-term fuel trim and long-term fuel trim (STFT and LTFT) values. Typically, fuel trims will be within 3% of the ideal, dependent on current air and coolant temperatures and load, for example.

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. The ECU will try to compensate, by reducing IPW, but if fuel trim goes over -25% or so, the ECU interprets this as a malfunction, setting a DTC (diagnostic trouble code), DTC P0172 – Fuel System Too Rich (Bank 1), and illuminating the MIL or CEL (malfunction indicator lamp or check-engine light).


What are the common causes of code P0172 ?

Depending on year, make, and model, DTC P0172 may have number of causes. Here are some of the most common.

  • Toyota / Lexus – Contaminated MAF, usually with aftermarket filters. The reasons for this correlation aren’t clear, but many Toyota hotwire MAFs are particularly susceptible to contamination. Cleaning isn’t usually successful. 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 P0172 ?

Depending on the nature of the failure, DTC P0172 may or may not be accompanied by drivability issues. At the very least, you will experience poor fuel economy, because, with the ECU in “limp-home mode,” it is no longer looking at HO2S and AFR sensors for feedback.

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 P0172 ?

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.

  • P0172 with P0175 or MAF DTCs – If you note both P0172 and P0175 or concurrent MAF-related DTCs, then you can likely rule out individual cylinder, ignition, and fuel injector problems. 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. replacement, along with a new air filter and properly sealed air box, would be a more permanent solution.
    • 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.
    • Verify proper ECT and IAT (intake air temperature) readings. The ECU may or may not pick up on stuck sensors, so it does one good to check that ECT and IAT are in normal ranges. For example, if you’ve been running the engine for half-an-hour on a hot day, the ECT should read at least 175 °F. If it still reads 32 °F, the ECU will enrich AFR to compensate, resulting in a rich condition.
  • P0172 with other Bank 1 DTCs – If you note only P0172, maybe concurrent Bank 1 DTCs, such as a cylinder misfire, focus your attention on Bank 1.
    • 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. Pay attention to concurrent DTCs, such as those pertaining to VVT (variable valve timing) or the ignition system.
    • 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.


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Codes Related to P0172

While diagnosing DTC P0172, you may also run across these other DTCs:

  • P0170 – Fuel Trim Malfunction (Bank 1)
  • P0173 – Fuel Trim Malfunction (Bank 2)
  • P0175 – Fuel System Too Rich (Bank 2)
  • P0101 – Mass Air Flow Circuit Range / Performance
  • P0104 – Mass Air Flow Circuit Intermittent
  • P030x – Cylinder Misfire Random or Specific

Along the same lines, but on the opposite end of the scale, other fuel system DTCs may include:

  • P0171 – Fuel System Too Lean (Bank 1)
  • P0174 – Fuel System Too Lean (Bank 2)

BAT Team Discussions for P0172

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