P0172: System Too Rich (Bank 1)

What is P0172?

Error code P0172 is an OBD-II diagnostic trouble code (DTC) defined as “System Too Rich (Bank 1).”

The P0172 error code is set when your Engine Control Module (ECM) or Powertrain Control Module (PCM) detects that there’s too much gasoline present in the air fuel mixture of your combustion chamber.

If you want to maintain maximum engine power and fuel efficiency, your air fuel ratio should be about 14.7:1. This air fuel ratio has the highest power output but the best fuel economy.

Your Engine Control Module and Powertrain Control Module have the ability to adjust your air fuel ratio through your oxygen sensors, air fuel sensor (AF sensor), and Manifold Absolute Pressure sensor (MAP sensor). However, they can only do so in small increments. 

So, if your air fuel ratio requires greater adjusting than normal, error code P0172 will be set.

Common symptoms

You may experience one or more of these common symptoms if you have an active P0172 code:

• An illuminated check engine light
• Worsened fuel economy
• Black smoke coming from your exhaust
• A strong smell of raw fuel from the exhaust
• Decreased engine power
• Engine hesitation on acceleration
• Rough idling
• Engine misfires
• Soot buildup on spark plug diodes

Note: Error code P0172 is commonly associated with code P0175, which indicates that your engine is running rich but refers to Bank 2.

Can I still drive?

Technically yes, but you shouldn’t do so for long.

Extended driving with an active DTC P0172 code can cause internal engine problems, and you could do damage to your spark plugs, pistons, and catalytic converters. 

Repairs or replacements to some of those components could add up to a rather costly amount. So, fixing this error code ASAP is best by heading to a professional mechanic.

P0172 causes

Here are some of the more common causes of check engine light code P0172:

• A dirty mass airflow sensor overstates how much air enters your engine’s combustion chamber. This may cause more fuel to be delivered to your combustion chamber than necessary.
• A clogged air filter means inadequate air enters your engine — causing it to run rich.
• A leaking fuel injector allows too much fuel into your combustion chamber.
• A vacuum leak means the air entering your engine isn’t controlled or metered by the throttle body. Unmetered air will lower the idle air count, and your engine will run rich.
• Worn-out spark plugs don’t ignite consistently, meaning unburnt fuel may be left in your engine.
• Too much pressure in the fuel from the high pressure fuel pump can result in fuel leaks, causing the fuel injector to deliver excessive fuel into the combustion chamber.
• A faulty oxygen sensor may misread how much air is entering your engine.
• A faulty fuel pressure regulator may be misstating the pressure of your engine.
• A problem in the coolant system like a broken thermostat or faulty coolant temperature sensor.

A defective purge valve stuck in an open position allows a lot of fumes into your engine — messing up your air fuel ratio.

Diagnosis

The diagnosis process could involve many components. Here’s how a mechanic will go about diagnosing the problem:

• First, they’ll perform a fuel pressure regulator test and check your fuel rail pressure sensor.
• Then, your mechanic will analyze your long term fuel trim and short term fuel trim values.
• If your engine is running rich, your fuel trim values respond by correcting to negative values.
• Next, they’ll use noid lights to check your fuel injector pulse.
• Then, they’ll inspect your vehicle’s PCV hoses, vacuum hoses, and intake manifold gasket for vacuum leaks.
• They’ll check your MAF sensor and oxygen sensor (O2 sensor) for issues.
• Then they’ll look at your air intake system for clogging.
• Finally, they’ll inspect your spark plugs.

It’s important to complete the entire diagnostic process before attempting to fix error code P0172. You don’t want to pay for a new oxygen sensor when a worn-out spark plug is the real cause of your system running rich.

Possible repairs for P0172 & Costs

Since check engine light code P0172 has so many possible causes, there isn’t just one solution to fix the problem.

Here’s how your mechanic will fix your active DTC P0172 code:

• Check for other error codes: Code P0172 doesn’t always show up in isolation, so checking and fixing other error codes which may be present is a good idea.
• Vacuum leak issues: Your mechanic will inspect your vacuum and PCV hoses for damage and replace them if necessary.
• MAF sensor and air filter issues: They’ll use brake cleaner to clean your MAF sensor and air filter or replace them if they’re damaged.
• Fuel system problems: Your mechanic will inspect your fuel lines for cracks, leaks, or pinches. Next, they’ll perform a fuel pressure test and clean or replace your fuel injector.
• Exhaust system problems: If you have an exhaust leak, your mechanic will replace your exhaust system.
• Coolant system issues: If there’s a problem in your coolant system, your mechanic will replace your thermostat or faulty coolant temperature sensor.
• Problematic spark plugs: If you have faulty spark plugs, your mechanic will replace them with new ones.

Before a professional mechanic can fix the problem, they’ll need about an hour of diagnosing time to determine the underlying issue. The cost of diagnosing the problem varies depending on where you live and the labor costs in your area, but you can expect to pay around $75-$150 for this service.

Once they’ve determined the problem, your mechanic will give you a repair estimate.

Here’s how much the repairs may cost, depending on the problem: 

• Vacuum leak: $100-$200
• Clean mass air flow sensor: $100
• Replace mass air flow sensor: $300
• New high pressure fuel pump: $1300-$1700
• Replace fuel pressure regulator: $200-$400
• New air fuel sensor or oxygen sensor (O2 sensor): $200-$300
• New spark plugs: $6-$30 per spark plug

Note: The repair costs mentioned above include labor charges.