Estimates Trouble Codes P0172

P0172: System Too Rich (Bank 1)

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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), also known as Engine Control Unit, detects too much gasoline in the exhaust gas coming out of your combustion chamber.

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

Your Engine Control Module has the ability to adjust the air fuel ratio through data from the oxygen sensor, air fuel sensor (AF sensor), and Manifold Absolute Pressure sensor (MAP sensor). However, the ECM can only change your airfuel mixture in small increments. 

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

Common Symptoms

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

  • An illuminated check engine light
  • Worsened fuel economy
  • Smoke or a strong smell of raw fuel from the exhaust
  • Decreased engine power
  • Engine hesitation on acceleration
  • Rough idle
  • Engine misfires 
  • Soot buildup on spark plug diodes

Note: Error code P0172 is commonly associated with trouble 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 converter

Repairs or replacements of some of these components are very expensive. So, the best way to fix this error code ASAP is by heading to a professional mechanic.

P0172 Causes

Here are some of the more common causes of code P0172:

A dirty mass airflow sensor misreads how much air enters your engine’s combustion chamber. This may cause more fuel to be delivered to your combustion chamber.

A clogged air filter means inadequate air enters your engine — causing it to run rich.

A leaking injector allows too much fuel into your combustion chamber.

A Worn-out spark plug doesn’t ignite consistently, meaning unburnt fuel may be left in your engine.

Too much pressure from the fuel pump can cause fuel leaks, as the fuel injector delivers excessive fuel into the combustion chamber.

A faulty oxygen sensor and air fuel ratio sensor is unable to monitor the exhaust gas and may misread the air fuel mixture delivered to the engine.

A faulty fuel pressure regulator may be misstating the pressure in the fuel system of your engine.

A problem in the coolant system, like a broken thermostat or faulty coolant temperature sensor, can lead to incorrect fuel delivery.

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


The diagnosis process could involve many components.
Here’s how a mechanic will go about diagnosing the P0172 trouble code:

1. Check the fuel pressure
Perform a fuel pressure regulator test and check the fuel rail pressure sensor.

2. Examine the fuel trim values
Use a scan tool to analyze the long term fuel trim and short term fuel trim values. If the engine is running a rich air fuel mixture, the fuel trim value responds by correcting negative values.

3. Check the fuel injector pulse
Use noid lights to check the fuel injector pulse.

4. Test the Mass Airflow and Oxygen sensors
Examine the mass air flow sensor (MAF sensor) and oxygen sensor (O2 sensor) for issues. Sometimes a faulty O2 reading could result from an exhaust leak near the sensor.

5. Inspect the air intake system
Check the air intake system for clogging.

6. Check for damaged spark plugs
Examine the spark plugs for any signs of soot on the spark plug tip.

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 a real cause of your system running rich.

Possible Repairs for P0172 & Costs

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

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

Check for other error codes: Your mechanic will check and fix other error codes because code P0172 doesn’t always show up in isolation.

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 the fuel lines for cracks, leaks, or pinches. Next, they’ll perform a fuel pressure test and clean your fuel injector (or replace a leaking injector.)

Exhaust system problems: If you have an exhaust leak, your mechanic will fix it or replace the exhaust system if needed.

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: Your mechanic will replace faulty spark plugs with new ones.

Before a professional mechanic can fix the trouble code, they’ll need about an hour of diagnosing time to determine the underlying issue. The cost of diagnosing the problem depends 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: 

  • New spark plugs: $6-$30 per spark plug 
  • Clean mass air flow sensor: $100
  • New engine coolant temperature sensor: $150-$200
  • Replace mass air flow sensor:$300
  • New air fuel sensor or oxygen sensor (O2 sensor): $200-$300
  • Replace fuel pressure regulator: $200-$400
  • New high-pressure fuel pump: $1300-$1700

Note: The repair cost estimates mentioned above include labor charges.

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