P0172 Code: Meaning, Symptoms, Causes, Fixes, And Repair Cost

If you're getting a P0172 trouble code and you're not sure what caused it, how you can fix it or what the repair cost is, then this guide is just for you.

Your car’s Check Engine light has come on, and you’ve identified that the ECU has stored the P0172 code. Okay. What now?

You should take your car straight to a mechanic. Yes, the situation isn’t immediately life-threatening, but the longer you leave it, the worse it’ll get. (This also means a larger repair bill.)

Don’t take any chances. Book your car in now.

In most cases, you’ll need to put aside a few hundred dollars for this repair (whatever it turns out to need).

In this guide, I will explain what the P0172 code is, including symptoms, causes, fixes, and some broad cost estimates.

Table of ContentsShow

What Does The P0172 Engine Code Mean?

car exhaust system at shallow depth of field

The P0172 code is defined as: System Too Rich (Bank 1).

In general, it means sensors have detected too much fuel (in relation to oxygen) in the exhaust pipe (and thus in the engine).

Remember, air and fuel are mixed in (or before) entering the cylinder. Thus, the P0172 code could also mean there’s too little air.

In the context of ratios, terminology doesn’t make that much difference. ‘Running rich’ means there’s too much fuel compared to the air. Another way of saying this is too little air compared with gasoline.

These mean the same thing.

Note: too much air compared to fuel (or too little fuel compared to air) is known as ‘running lean’.

What Is Bank 1?

“Bank 1” refers to the exhaust pipe side (left or right).

In V-engines (such as a V6 or V8), “banks” of cylinders make up each half of the “V” shape. For example, in a V8, each bank contains four cylinders.

Each bank has its own exhaust pipe leaving the manifold. This is good for flow and means you can identify on which side of the engine any problems develop.

These two banks are (creatively) referred to as Bank 1 and Bank 2. Bank 1 is the exhaust system, cat, and oxygen sensors attached to the bank containing cylinder #1.

Bank 2 is just the other one.

Every manufacturer numbers cylinders differently. Check your owner’s manual to find which one it is. Once you know this, you’ll have found Bank 1.

Why Your Car Has A P0172 Code

Catalytic Converter

Two oxygen (otherwise known as O2 or lambda) sensors are located in each exhaust pipe. One is before the catalytic converter (upstream, aka Sensor 1), while the other is after it (downstream, Sensor 2).

The oxygen sensors’ combined job is to monitor the performance of the cat. This, in turn, removes pollutants and hydrocarbon particulates (unburnt fuel, or soot).

The readings from the oxygen sensors are sent to the ECM, part of the ECU. Here, the system analyzes them. If the readings show it’s necessary, the Check Engine light illuminates, and the DTC is stored.

The P0172 code indicates that the catalytic converter can’t remove all the hydrocarbons from the air simply because there’s too much of it.

Unburnt fuel, of course, comes from the engine. In this case, it comes from the bank containing cylinder #1 (Bank 1). One or more of these cylinders are ‘running rich’.

Symptoms Of P0172 Code

When an engine trouble code is stored, the Check Engine light illuminates. An OBD II reader (code scanner) can reveal what the ECU thinks has happened.

Aside from the Check Engine light and the stored code (only visible via the scanner), you might notice any of the following:

  • A strong smell of fuel (like when starting your car) all the time
  • Misfires
  • Low power, especially under heavy loads (hard acceleration, fast speeds)
  • Black-tinged smoke from your exhaust
  • A rough idle
  • Poorer fuel economy than usual
  • Poor emissions test performance (including failure)

These all stem from the rich fuel ratio. Too much fuel in the chamber means the combustion isn’t as efficient as it should be. This translates into all the symptoms mentioned above.

P0172 Diagnostic Trouble Code Causes

Oxygen sensor bank 1 bank 2 sensor 1 sensor 2

Almost anything could be causing a rich fuel mixture. The most common are some of the easiest to fix:

  • Faulty or soot-covered oxygen sensor(s)
  • Damaged spark plugs or spark plugs covered with hydrocarbon deposits (soot)

New spark plugs are a straightforward job in most cases (Boxers and certain V-engines aside). Oxygen sensors are usually relatively easy for a mechanic, too.

If these causes are eliminated, the list of possible causes widens considerably. The issue could lie anywhere in the ignition or emissions systems.

You should get the following checked out, although this is by no means an exhaustive list:

  • Fuel injectors
  • Fuel lines
  • Fuel pump
  • Fuel filter
  • Fuel pressure regulator
  • MAF sensor
  • MAP sensor
  • Throttle body
  • Throttle position sensor (TPS)
  • Air filter and intake boot
  • Coil packs
  • Catalytic converter
  • ECU
  • Vacuum leak

How To Fix A P0172 Code

As with most emissions-related problems, the most effective solution is to go to a mechanic. They’ll have seen this sort of thing hundreds of times and can use advanced scanners to zone in on the problem.

Usually – but not always! – a new oxygen sensor or set of spark plugs fixes the problem.

Mechanics should use testing equipment to find the component(s) that isn’t working. This can then be replaced.

It’s also a good idea to use cleaning services and additives. Sea Foam in the fuel tank helps clean the injectors, while a carbon cleaning service could clear out your exhaust system.

How Much Does A P0172 Code Cost To Fix?

Dollars (Price, Cost)

There’s no easy answer to this. Fixing a trouble code is more complex than buying a single new part.

The price you’ll pay is entirely dependent on the identified necessary repair.

Here are a few (broad and approximate) general estimates. Remember that you might need more than one of these!

  • Spark plugs – $100
  • Oxygen sensor – $350
  • Fuel pressure regulator – $300
  • Coil packs – $250 (for a whole set)
  • MAF sensor – $100
  • MAP sensor – $200
  • Fuel system service – $80
  • Carbon cleaning service – $300
  • ECU inspection and reprogramming – $500 to $1,500
  • Engine replacement – $3,000+

Although there are some big numbers here, don’t lose hope just yet! Wait to see what the mechanic tells you.

Concluding Thoughts On The P0172 Code

The Check Engine light may be annoying to some, but it’s there for a reason. Literally.

Don’t ignore it! The longer these things are left, the worse (and the more expensive!) they become.

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Benjamin Kitchen

Ben is an automotive author from England. With experience in a fast-fit garage, he's an IMI-qualified light vehicle technician. He aims to help drivers worldwide with common automotive problems. You’ll often find him working with his 1.2 Vauxhall Corsa – it may have a tiny engine, but in eight years it's never once let him down!