Codes P0420 stored and P2173?

RROLSTON14

MEMBER
2012 JEEP COMPASS
2.4L
4 CYL
4WD
AUTOMATIC
165,000 MILES

So, for the past 4 months in New England, it’s obviously winter and anytime I start my car in the morning and it’s below 40 while it’s warming up the red lightning bolt comes on the engine RPM revs high and I shut it off and turn it back on and it’s fine. Doesn’t do it again until the next morning. I recently replaced the alternator because that stopped working. I’ve replaced the spark plugs, the coils, the o2 sensors, the MAP sensors, and the TPS sensor. And I have an obd2 scanner I can provide the readings I'm getting for whatever will help. I don’t understand them but I’m trying to learn. Any help is much appreciated.

7 Replies

JACOBANDNICKOLAS

MECHANIC
108,269 POSTS

Hi,

The P0420 indicates the catalytic converter is operating below the threshold. The P2173 is indicating an engine vacuum leak. The two can be related.

First, I need you to check something for me. If your scan tool has live data, check the short-term fuel trims when the engine is at operating temperature and at idle. Let me know what they are.

If you don't have that option, start by checking for possible engine vacuum leaks. Also, check the PCV valve and hose to make sure they are connected on both ends and the hose isn't cracked. See pic below.

Here is a link that explains how to check for a vacuum leak:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

One last thing. Confirm there are no exhaust leaks before the catalytic converter.
'
Let me know what you find or if you have questions,

Take care,

Joe

See pic below.

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Saturday, April 13th, 2024 AT 7:29 PM

RROLSTON14

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So, I just checked the live data and at idle in park the short-term fuel trim is bouncing between -3.1 and -.8% and I don’t know what that means.

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Sunday, April 14th, 2024 AT 8:37 AM

RROLSTON14

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Thank you for your response as well. I am going to check the other 2 things you mentioned now. I will definitely let you know.

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Sunday, April 14th, 2024 AT 8:38 AM

RROLSTON14

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Here are the live data freeze frame results.

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Sunday, April 14th, 2024 AT 8:45 AM

JACOBANDNICKOLAS

MECHANIC
108,269 POSTS

Hi,

Thanks for the information. The short-term fuel trims (STFT) can be used to determine if the engine is running too rich or too lean. (Too much or too little fuel). In this case, the negative number indicates the computer is trying to reduce the amount of fuel being used. That is the opposite of what I would have expected for an engine vacuum leak.

In a perfect world, the STFT would be 0% with little fluctuation. However, up to 10% + or - is considered fine and you are within those specifications.

Have you been able to check the other things I mentioned, including an exhaust leak?

Let me know.

Joe

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Sunday, April 14th, 2024 AT 7:51 PM

RROLSTON14

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5 POSTS

I remember when I replaced the upstream o2 sensor and I went to put the 3 bolts back into the cover that went on it I’m assuming that’s a heat shield. And there was nothing for me to screw into. I used a ratchet to get them out, and I put the plate back on and whatever was underneath it had fallen off from rust damage. But the problems were there before that I just don’t know if it made it worse. I will check those other 2 things after work today.

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Monday, April 15th, 2024 AT 9:15 AM

JACOBANDNICKOLAS