Check Engine Light Keeps Coming Back… Any Ideas?

Subaru Forums Subaru Forester
1

So here’s what’s going on: My check engine light keeps coming on and off. When I reset the ECU, it clears for a while, but after a few days, it comes back. I’m also getting warnings about the emission control system and hill-start assist not working.

I’ve already tried the basics, like tightening the gas cap and resetting the ECU, but no luck. The car drives fine otherwise.

I took it to a Subaru service center. They replaced the O2 and AFR sensors, ran diagnostics, and handed me a big bill. But within two minutes of leaving, the check engine light came back on.

I feel like if I go back, they’ll just replace more parts and charge me again. Any advice on what to check or do next?

Details: 2016 XT 2.0. Error code is P0172: “System Too Rich, Bank 1.”

2

Did you get the exact code?

3

LisaMoore said:
Did you get the exact code?

Yeah, it’s P0172: “System Too Rich, Bank 1.”

4

Zan said:

LisaMoore said:
Did you get the exact code?

Yeah, it’s P0172: “System Too Rich, Bank 1.”

Did they check the mass air flow (MAF) sensor before replacing the AFR sensor? Have you done any repairs recently?

5

@LisaMoore
No recent repairs. They didn’t mention the MAF sensor. Honestly, I wouldn’t be surprised if that’s the next thing they try to sell me.

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Zan said:
@LisaMoore
No recent repairs. They didn’t mention the MAF sensor. Honestly, I wouldn’t be surprised if that’s the next thing they try to sell me.

Are you noticing any driving issues? Smelling fuel or anything unusual?

7

@LisaMoore
Nope, nothing unusual. No fuel smells, and the car drives fine. When it’s cold, the idle is high, but I think that’s normal.

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@LisaMoore
When’s the last time you changed the air filter? Also, did you check for vacuum leaks?

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LisaMoore said:
@LisaMoore
When’s the last time you changed the air filter? Also, did you check for vacuum leaks?

Changed the air filter 7 or 8 months ago. Haven’t checked for vacuum leaks yet but started watching YouTube videos to figure it out.

10

Is it P0420 by any chance?

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Audrey said:
Is it P0420 by any chance?

No, it’s P0172: “System Too Rich, Bank 1.”

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Zan said:

Audrey said:
Is it P0420 by any chance?

No, it’s P0172: “System Too Rich, Bank 1.”

Got it. A lot of people replace the AFR and O2 sensors for this code, especially on the 2.5 engines.

Any chance you’re near Sacramento? We could take a look: link.

13

@Audrey
Mine’s the 2.0, and I’m way up in Northern Canada.

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Zan said:
@Audrey
Mine’s the 2.0, and I’m way up in Northern Canada.

Gotcha. I’m surprised they replaced the AFR sensor instead of checking the MAF first.

For the 2.0, it could also be a high-pressure fuel pump leaking gas into the crankcase. Or maybe even leaking injectors.