P0175 – Is Your Engine Running Too Rich (On Fuel)?

Paul Hadley

This article was recently reviewed by car expert Paul Hadley to ensure it is as accurate and helpful as possible.

Automotive engines are incredibly precise and intricate pieces of machinery, with a minimal margin of error to account for. For every combustion, a precise amount of fresh air and fuel is pumped into your engine for ignition to take place. If it’s thrown out of equilibrium by even a microscopic amount, it can severely risk your car’s driveability, performance, and reliability. Such is the case with P0175.

As we all know, combustion requires two key ingredients – fuel (be it gasoline or diesel), and air (fresh oxygen from the atmosphere). In every engine, there’s such a thing as an air-to-fuel ratio. This denotes how many parts of air are mixed with a corresponding part of fuel, for thorough combustion to occur. When your air-to-fuel ratio is no longer within its threshold, it risks running lean or rich.

The latter is pertinent when it comes to a P0175 error code that your car spews at you. When it starts to run “rich”, it simply means that there’s too much fuel for the amount of air within the combustion chamber. An excess here equates to unburnt fuel. In other words, affecting your fuel economy, not to mention causing havoc as far as the long-term wearing of the engine goes. So, what’s all about P0175?

What Is This Air-To-Fuel Ratio, Anyway?

So, we made mentions of an “air-to-fuel ratio” earlier, and its relations to a P0175 diagnostics trouble code. So, what is this ratio, and why does it matter? In general, an ideal air-to-fuel ratio would follow stoichiometric measurements of 14.7:1. Or to put it another way, complete combustion should see a mixture of 14.7 parts of air (measured in grams) for every 1 part of fuel (also measured in grams).

For example, let’s say there are 29.4 grams (that’s 14.7 x 2) of air rushing into the combustion chamber. To ensure a performant, efficient, and emissions-friendly burn, 2 grams of fuel are pumped into your engine. An engine’s central module, its ECU or ‘engine control unit‘ (aka an ECM or ‘engine command module’, or PCM or ‘powertrain control module‘) keeps an active lookout to continually balance this.

It does this through a myriad of sensors and actuators. The ‘mass airflow’ or MAF sensors work hand-in-hand with the oxygen (O2) sensors. In doing so, they’re able to measure the precise amount of air going into the engine, while the latter monitors the exhaust fumes. Both of which, which can work in conjunction with other inputs like the MAP or BARO sensors, send airflow data back to the ECU.

With detailed analysis and onboard computation, the ECU now knows how much air there is. It could then prompt your car’s fuel injection system to inject (read: pump in) a precise amount of fuel into the combustion chamber, corresponding with the volume of air. Note, not all cars follow the 14.7:1 ratio perfectly. It’s tuned on an engine-to-engine basis, as diesel carries a lower 14.5:1 ratio on average.

What Would The Wrong Air-To-Fuel Ratio Be?

So, and as we’ve learned so far, your car’s air-to-fuel ratio strays close to the 14.7:1 range. Some may carry manufacturer-specific ratios that are more or less close to that. But what would an incorrect air-to-fuel ratio be? In summary, “rich” means that there’s too much fuel and not enough air. Vice versa, a “lean” mixture tells you that there’s too much air, and not enough fuel for a complete burn.

Therefore, either rich or lean will skew the stoichiometric ratio. Just as an example, you could expect something along the lines of:

  • Rich – Approx. 13:1 air-to-fuel ratio. With (a lot) more fuel in the system than is ideally required, you can expect a burst of power. Albeit, the extra performance isn’t optimized as best as it can. Unburnt fuel will drop your fuel economy figures substantially, while also increasing tailpipe emissions.
  • Lean – Approx. 16:1 air-to-fuel ratio. This time around, there’s far too much air in the combustion mix when compared to fuel. With less fuel being burnt, you should expect better fuel economy and lower tailpipe emissions. However, your car’s performance will suffer badly, contrary to running rich.

Plus, it’ll vary further depending on the type of engine:

  • Spark-Ignition (Gasoline Engines) – The term ‘spark’ comes from the spark plugs that ignite the air and fuel mixture inside the combustion chamber. With Spark Ignition (SI) engines, the range is commonly around 6:1 for a rich mixture, all the way up to 20:1 for a lean mixture.
  • Compression-Ignition (Diesel Engines) – Unlike gas engines, diesel powertrains rely mostly on heat and compression to ignite, rather than a concentrated spark. Using Compression Ignition (CI) engines, the range is typically between 9:1 for a rich mixture or up to 70:1 for lean.

Does The Air-To-Fuel Ratio Remain Constant?

As we highlighted earlier, air-to-fuel ratios differ from one engine to the next. Additionally, you’ll also have to consider the type of fuel you use. Gasoline and diesel are the most common, with an ideal air-to-fuel ratio rated at 14.7:1 and 14.5:1 for both of them, respectively. But how about other fuels used out there? Varying types or compositions of fuels will require differing amounts of air to burn properly.

Alternatives to gasoline and diesel are becoming commonplace, so it’s worthy of discussion. Cars are rapidly adopting the use of biofuels. While they’re mostly just regular gasoline or diesel, they can mix in a bit of methanol, ethanol, or butanol. Mixing them together would chemically help to improve fuel economy and reduce emissions. For a comparison of their stoichiometric ratios, we have:

Engine air to fuel ratio running rich

  • Methanol – 6.47:1
  • Ethanol – 9:1
  • Butanol – 11.2:1
  • Propane – 15.67:1
  • Methane – 17.19:1
  • Hydrogen – 34.3:1

Interestingly, an engine isn’t able to maintain a perfect air-to-fuel ratio at all times. Even with all that tech, constantly balancing that ratio is extremely challenging, if not impossible. For that reason, your engine tends to flip-flop between lean and rich constantly in the background. When you’re starting up your car early in the morning, your ECU will run and manage the engine in an ‘open loop’ manner.

This means that the air-to-fuel ratio is fixed, unmanaged, and is biased toward being richer. Thus, you’ll surely notice higher emissions and fuel consumption just after you’ve cranked the engine. But once all those sensors get up and running, your ECU will then switch to a ‘closed loop’ operation. Here, an ECU would begin to carefully flip-flop between rich and lean, averaging it to be as balanced as possible.

How Does The Air-To-Fuel Ratio Vary?

Throughout your car’s workday, and depending on what you’re doing with it, the air-to-fuel ratio will constantly change. Thus, it’s usually never a 14.7:1 mixture all the time. For a spot of context, here’s what your air-to-fuel ratio looks like based on the state of your engine:

  • Cranking Up (As Low As 9:1) – Since the engine and its many components are cold, a lot of fuel needs to burn to get it warmed up. Hence, your engine runs extremely rich.
  • Warming Up (Around 12:1) – Once the engine’s started, it’ll take a bit more time before it gets up to its operating temperature. To keep it going, it’ll continue to run rich, but less so.
  • Acceleration (11:1 To 13:1) – More throttle input means more fuel is burned. With performance gains, you’ll be running at a fairly rich 11:1 ratio. But be more modest with the gas pedal, and it could drop to a still-rich 13:1.
  • Deceleration (Around 17:1) – When you’re decelerating, it means that you’re not putting your foot on the gas pedal. The engine doesn’t need any more fuel than absolutely necessary to keep it running. In that sense, it could run extremely lean.
  • Cruising (Around 14.7:1) – At this stage, the engine is already hot, and you’re traveling at a constant speed (with no excess throttle input). As such, the ECU could manage the air-to-fuel ratio to its ideal stoichiometric sweet spot.
  • Heavier Load (Around 12:1) – But let’s say you’re hauling a lot of cargo, a cabin filled with passengers, or are going uphill and at speed? Once again, with more power needed, your engine will run slightly richer, at 12:1 or thereabouts.

What Does A P0175 Error Code Mean?

So, how then are you alerted to an imbalanced air-to-fuel ratio in your engine? Well, when the ECU can tell that something’s amiss from odd readings taken by the many sensors inside a vehicle. It’ll thus prompt an error message to appear. In the case of an incorrect air-to-fuel ratio, that lends more towards the richer side of things, a P0175 error code will come up once scanned with an OBD tool.

The accompanying error message to a P0175 OBDII diagnostics trouble code (DTC) would read along the lines of, “System Too Rich (Bank 2)”. Thus, entailing that your engine’s air-to-fuel ratio is fairly rich in the mixture. Meanwhile, “bank 2” refers to which bank of cylinders it’s specifically running richer in. For an engine with a V-layout (e.g. a V6, V8, etc. engine), we can refer to ‘bank 1’ and ‘bank 2’ as:


  • Bank 1 – Cylinders that have odd numbering, such as Cylinder 1, 3, 5, 7, …
  • Bank 2 – Cylinders that have even numbering, such as Cylinder 2, 4, 6, 8, …

You’ll have to refer to your owner’s or service manual to find out where each cylinder is if you want to further diagnose a P0175 issue. Although, inline-type engines will also use ‘bank 1’ and ‘bank 2’, even though they don’t have separate cylinder banks like V engines. Instead, an inline (straight) engine will split up the cylinder count in half (say, 3 cylinders each “bank” for an inline-6), so you’ll see it as:

  • Bank 1 – The first-half of the cylinder count, cylinders 1, 2, and 3 (or just 1 and 2 for an inline-4)
  • Bank 2 – The second-half of the cylinder count, cylinders 4, 5, and 6 (or just 3 and 4 for an inline-4)

What Causes A P0175 Error Code To Appear?

Besides P0175, System Too Rich (Bank 2), there are also other OBD trouble codes that relate to your engine running rich. In particular, a P0172 error code, or “System Too Rich (Bank 1)”. The latter would appear if your engine’s ‘bank 1’ is running richer than usual, while ‘bank 2’ is nominal. Regardless, an engine running richer than it should be could be caused by many root issues:

  • Strictly limiting the oncoming airflow and intake of oxygen, hence causing less air to enter the engine to mix with fuel for thorough combustion.
  • Injecting and pumping far too much fuel into the engine, more so than what’s required, thus ensuring that what little air there is can’t burn it all.
  • Sensors that are responsible for continuously measuring and providing real-time feedback on the air-to-fuel ratio are providing false positives or incorrect readings.

Between those 3 causes of why your air-to-fuel ratio is running rich, the key points of failure include:

What Are The Symptoms Accompanying P0175?

Should your engine run rich, as is the case with a P0175 diagnostics code, there ought to be numerous symptoms to accompany it. With an air-to-fuel ratio that’s been thrown out of whack, the driveability and performance of your car will begin to suffer. Thus, making it relatively easier for you to spot what might be wrong. So, be on the lookout for these symptoms of a P0175 error code:

Engine air to fuel ratio running rich

At first, a P0175 error code (engine running rich) might not impact your car’s driveability right away. However, it’ll get progressively worse, especially once the engine’s been warmed up to its operating temperatures. There, you’ll start to notice rough idling, lack of power, hesitation while accelerating, cylinder misfiring, or stalling while driving more prominently than before.

How Can You Diagnose (And Fix) A P0175 Error Code?

Before we move towards more expensive solutions for a P0175 error code, there’s always a chance for us to possibly fix it through a detailed diagnosis. At the very least, we also get the chance to properly narrow down the component(s) that have failed or gone awry. Hence, enabling us to accurately count how much we’ll need to spend getting it fixed. So, how can we diagnose a P0175 issue further?

Moreover, how serious is this issue, anyway? Well, having your engine running rich, as we’ve detailed earlier, is quite normal. For the most part, it’s a regular aspect of an engine’s daily workload, as they alternate between rich and lean constantly, as the situation demands it. Nevertheless, intentionally running richer than ideal, while okay in the short-term, will be damaging if not resolved promptly.

At the very least, you’ll be burning away more fuel. Thus, resulting in higher fuel consumption, with more frequent fill-ups required. Beyond that, an overly rich air-to-fuel ratio will compound its issues down the line. It can clog up the catalytic converter and other internal components within the engine with carbon build-up. This alone could cost you thousands in repairs and replacements if they fail.

Worse, a rich-running engine can scrap itself in time, causing irreparable internal damage within the engine itself. We’re talking about scoring up the pistons and cylinder walls, or further wearing out an engine block. In either scenario, it’ll significantly shorten the lifespan of your engine, accelerating the need for a thorough rebuild or replacement. Once again, costing you thousands to fix.

Step 1: Scan For Any Other Error Codes (Apart From P0175)

Grab an OBDII diagnostics tool, and scan the ECU for any other error codes. Should there be trouble codes other than P0175, you should diagnose and fix (repair or replace) those ones first. Once you’ve gone through them, refresh all error codes detected on the system, and see if P0175 reappears.

If that’s the case, then you’ve yet to fix the engine running rich. In particular, pay close attention to error codes that relate to components that could’ve caused the engine to run rich in the first place. As we listed earlier, components such as:


Step 2: Conduct A Visual Inspection Around Your Engine

It’ll be handy to have a repair manual here. You can refer to it as you navigate around the engine bay, looking for the aforementioned components that could cause a P0175 error code to appear. Now, we can inspect these components to see if they have failed:


  1. Check all the vacuum hoses, as well as the PCV (positive crankcase ventilation) hoses and valves. See if there’s a crack or leakage in the hoses.
  2. Inspect all the fuel lines for any leaks, cracks, or pinching.
  3. Check the fuel pressure regulator near the fuel rails, to see if they’re leaking fuel.
  4. Look at the oxygen sensor near the exhaust manifold, and see if there’s a vacuum (air) leak.
  5. Look over the coolant temperature sensor and thermostat, to ensure they’re in working condition.
  6. Remove and inspect the fuel injectors. If they’re dirty or clogged up with debris and carbon build-up, you can use a special fuel injector cleaner to give them a wash.
  7. Remove and check the spark plugs, and look near the tip if there are scoring or burn marks. You can try swapping the spark plugs around from one cylinder to the next to see how the engine runs.
  8. Locate, disconnect, and scan the condition of the MAF sensor (as well as MAP and BARO, if needed) to see if it’s clogged up. Should there be debris there, use a dedicated electronics cleaning solution to clean it.
  9. Go through the air intakes and intake manifold, and make sure there’s not a lot of debris that could clog up the airflow there.
  10. Make sure you’re not using an aftermarket air filter, which could sometimes skew the MAF readings.

Engine Diagnostic Trouble Code (DTC) Facts:

  • P0171 code appears when the engine runs lean due to too much air or too little gasoline, while P0175 code appears when the engine runs rich due to too much fuel or too little air.
  • A vacuum leak or a poor fuel system can cause a rich state, and the powertrain control module pumps more fuel to maintain the required air-fuel ratio.
  • Symptoms of P0171 and P0175 include a check engine light turning on, rough idling, engine misfiring, and strong odor from the exhaust.
  • Causes of P0175 include clogged or leaking fuel injector, fuel regulator failure, defective mass air-flow sensor, thermostat malfunction, and issues with fuel delivery or exhaust.
  • An excessively rich system may reduce the lifespan of the catalytic converter and increase gas consumption, leading to higher levels of pollutants.
  • While driving with a P0175 code is possible, ignoring the code may result in an inefficient car and more serious problems in the long run.
  • To diagnose P0175, a mechanic inspects fuel pressure, injectors, lines, oxygen sensors, and engine temperature.
  • Common mistakes while diagnosing include not measuring the engine temperature and considering a component defective without testing it.
  • To fix P0175, one may need to replace vacuum lines, O2 sensors, mass air-flow sensor, fuel pump, fuel filter, fuel injector, thermostat, or coolant temperature sensor.
  • The estimated cost of repairs for P0175 varies from $100 to $1700 depending on the issue, location, and car type.

Final Thoughts On A P0175 Error Code

If any of the abovementioned components have failed or are showing signs of failing, then you should replace them where necessary. But in some instances, clogged up or dirty parts (like sensors) could be cleaned up, rather than repaired or replaced. Sensors usually cost hundreds to replace, depending on how accessible they are. Meanwhile, fuel injectors and other fuel-related issues may cost you more.

You could sometimes expect repair costs leading up to $1,000, $1,500, or higher, based on what issue you have at hand. Nonetheless, this is far cheaper than rebuilding the entire engine, around twice to three times as much. While an imbalanced air-to-fuel ratio might seem fairly innocent or tame at first, it could cause serious problems with your engine. So, be sure to get it inspected and fixed promptly.

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