Ford P0172, P0175

Bad purge valve sets Ford P0172, P0175 trouble codes

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 set Ford P0172, P0175 trouble codes

What Ford P0172, Ford P0175 trouble codes mean

Let’s start with the official definition:

P0172 System too Rich (Bank 1)

P0174 System too Lean (Bank 2)

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

What causes a rich condition? P0172, P0175

The possibilities are 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 (which really doesn’t make a lot of sense that you’d have a leaking fuel injector on both banks of a V engine), or something else that’s allowing too much gas into the mixture. That something else would have to be AFTER the MAF sensor.

Why think it’s caused by a bad purge valve?

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

Charcoal vapor canister

The MAF doesn’t sense this added air/fuel and neither does the computer. So you’ve got a situation where extra gas is being poured onto the fire. The computer is reacting 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 written a post on it. Find it here.

How to check the purge valve

Once you locate the purge valve, disconnect the vacuum line coming from the charcoal canister. Look for debris in the line. If you find charcoal, you’ll have to replace the both 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 make sure you aren’t finding vacuum on the charcoal side of the valve with the engine running. Just 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, 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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