Rick's Free Auto Repair Advice

P0172 code: What it means and the most common causes

Learn what a P0172 code means, the most common causes and how to fix it

A P0172 code is defined as System too Rich (Bank 1). Bank 1 is the bank of cylinders that contains #1 cylinder. The code is telling you that it’s seeing a constantly rich reading from the oxygen sensor located in the Bank 1 exhaust stream.

The most common symptoms of a System Too Rich Code

• Check engine light
• Poor idle
• Excessive fuel smell from the exhaust
• Black soot inside the tailpipe• Low power
• Engine misfire
• Stalling
• Hesitation
• Hard start, especially a restart after the engine is warm
• No start

The most common causes of a P0172 code

• Faulty MAF sensor
• Leaking fuel injector — leaks fuel into the cylinder(s) even when shut off
• High fuel pressure — Too much fuel enters the cylinder during each fuel injector ON cycle
• Faulty engine coolant temperature or ambient air temperature sensor— The sensor is reporting a lower coolant temp than actual (The coolant temperature sensor reading will default to its coldest reading if there’s an open in the wiring harness or the ECT connector is disconnected). The ECM will think the engine is cold and will command a rich mixture).
• Plugged engine air filter — much rarer than the causes listed above. but worth checking

How to diagnose P0172 and P0172

Tools you’ll need

• Scan tool with live data capabilities
• MAF sensor cleaner
• Flashlight
• Multimeter
• Fuel pressure gauge
• Thermometer

Start by reading O2, MAF, and short-term fuel trims

Set the scan tool to live data and monitor the operation of the upstream O2 sensor(s), MAF, and short-term fuel trim readings.

Clean the MAF sensor

The Mass airflow sensor (MAF) is located between the air filter box and the throttle body. It can be mucked up with leaves and debris, and the hot wire can become coated with oil. Clean the MAF sensor using MAF sensor cleaner. Here’s a step-by-step article on how to clean a MAF sensor. 

After cleaning, check MAF data on the scan tool. If the reading changes, chances are the MAF sensing wire was dirty.

A MAF reading of 1 gram per second per engine liter at idle is acceptable. So, at idle speed, a 4.0-liter engine would read approximately 4 grams per second.

Check the temperature sensors for accuracy

Using a contact thermometer near the thermostat, compare engine temperature readings from the ECT to the actual engine temperature. If the reading is off, check for an open in the sensor wire or a disconnected connector.

Repeat the test for the ambient air temperature sensor

Repair or replace the sensors/wiring if necessary.

Check fuel pressure

Rent a fuel pressure gauge from an auto parts store and connect it to the fuel rail. With the engine off, turn the key to the RUN position. The fuel pump should prime, and the pressure should rise to the spec in the shop manual. If it does, then start the engine and check to see if the fuel pump maintains the correct pressure.

If the pressure is lower than the spec, the fuel pump is not delivering the amount of fuel commanded by the ECM, and that can set a P0171 or P0174 trouble code. Determine if the cause is the fuel pressure regulator or the fuel pump. Refer to the shop manual for regulator testing.

Check for leaking fuel injectors

Connect the fuel pressure gauge as above and turn the key to the RUN The image shows leaking fuel injector a that can cause a P0175position to prime the fuel system. Then turn the key to OFF and watch the pressure. A pressure drop of 5 psi is acceptable. If the pressure drops more than that, you may have leaking fuel injectors or a faulty check valve in the pump. To determine which is which, remove the spark plugs and check for signs of fuel in the cylinder. A wet spark plug is a sign of a leaking fuel injector. You can try adding a fuel injector cleaning solution to the tank or have a shop perform a more intensive cleaning. But in many cases you may have to replace at least one injector to eliminate the P0172, P0172 code.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

Custom Wordpress Website created by Wizzy Wig Web Design, Minneapolis MN