Understanding a P0172 or P0175 code on a Ford

Learn how to fix a P0172 or P0175 code on a Ford

Purge valve

These trouble codes can have you chasing your tail down all kinds of dead ends. But many times, a bad purge valve can cause a P0172 or P0175 code on a Ford vehicle.

What a P0172 or P0175 code means

Let’s start with the official definition:

P0172 System too Rich (Bank 1)
P0174 System too Lean (Bank 2)

The codes tell us that this is a “V” style engine because it shows codes for two engine banks, and both oxygen sensors detect a rich condition in the exhaust before the catalytic converter.

What causes a rich condition and codes P0172 and P0175

Here are some common causes of these trouble codes:

• a bad or dirty MAF sensor
• a vacuum leak that’s causing the computer to over-correct and add too much fuel to compensate
• a leaking fuel injector (it doesn’t make much sense that you’d have a leaking fuel injector on both banks of a V engine at the same time)
• or something else that’s allowing too much gas into the mixture. That something else would have to be AFTER the MAF sensor.

Why it’s often caused by a bad purge valve on Ford products

• Fords have a high failure rate on the purge valve— They tend to stick in the open position. That allows them to suck fuel vapor from the charcoal canister constantly.

Charcoal vapor canister

The MAF doesn’t sense this added air/fuel, and neither does the computer. So you’ve got a situation where unmetered extra gas is being sucked into the manifold and added to the cylinder. The computer reacts to the rich mixture being reported by the oxygen sensors by cutting fuel from the fuel injectors and it’s still not helping. So, it sets the code.

The purge valve is part of the evaporative emissions system. If you don’t know how this system works, I’ve posted on it. Find it here.

How to check the purge valve

Once you locate the purge valve, disconnect the vacuum line from the charcoal canister. Look for debris in the line. If you find charcoal, you’ll have to replace the valve and the charcoal canister and clean out the vacuum line. You can buy replacement canisters from aftermarket suppliers—you don’t have to go to the dealer. If you don’t find charcoal, check to ensure you aren’t finding a vacuum on the charcoal side of the valve with the engine running. Start the engine with the line removed and place your finger over the valve opening. If there’s suction, the valve is stuck open. Replace it.

How to reset the PCM, ECM

Disconnect both battery cables and touch them together for about 30 seconds. That will drain the capacitors in the PCM and ECM and erase all the keep-alive memory. The vehicle will behave differently for the first 10 starts as it relearns the condition of your engine, but the codes should not return.

©, 2015 Rick Muscoplat

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