× Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!

jeep liberty 2004 3.7 trouble codes 0172 and 0171 at the same time

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23 Mar 2019 19:08 #28048 by juergen.scholl

overdrive138 wrote: when u say compare pre and postcat o2 what exactly am i comparing? thx


You compare the voltage readings of the 2 sensors, the one before the catalytic converter and the one behind it....(of the same bank, obiously).

The front one should show an oscillating or fluctuating signal somewhere between 100 mV and 900 mV . This would indicate that the computer has got fuel control and is able to alter the mixture in order to feed the converter with the proper air/fuel mixture to make it work.

The second sensor, the one behind the catalytic converter, should show a much more stable voltage reading if the converter itself is working properly. This voltage may read around 300mV or 700mV, depending on design and system. With your new verus you can compare the sensors' voltages easily in graphic mode.

If the second sensor shows fluctuating voltages similar to the pre cat sensor - in other words the second sensor is mirroring the first one - then the converter is not doing its job, hth.

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24 Mar 2019 17:48 #28058 by Matts Auto
Rough idle could be dirty throttle plate/housing

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27 Mar 2019 08:28 #28153 by rpd1125
I had one of these with exact code. Its just running lean on a centralized part of the intake manifold, meaning a vacuum leak. Take a smile generator and put to vacuum port and fill it up ....usually shows up very quickly but what i found was the the Purge control solenoid was stuck wide open allowing full vacuum to the vapor line to the fuel tank..

Easy test is to remove supply vapor line to the Purge control solenoid which is usually mounted right near or intake or driver side fender, easy test is turn the key on, turn on the smoke generator and if any smoke comes out of the solenoid out of the unplugged port to fuel tank there is your problem.

Almost any scanner will allow you the test ability to perform a purge control solenoid test and it allows you to cycle it in 10% increments and should allow you to see the duty cycle while performing this test. The Purge control solenoid should be completely closed at idle and with KOEO. same test can be performed on just about any chrysler or dodge vehicles. Again very easy to find it, your computer may also show Large evap system leak.


Hope that helps
The following user(s) said Thank You: Andy.MacFadyen

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27 Mar 2019 10:32 #28154 by Andy.MacFadyen

rpd1125 wrote: I had one of these with exact code. Its just running lean on a centralized part of the intake manifold, meaning a vacuum leak. Take a smile generator and put to vacuum port and fill it up ....usually shows up very quickly but what i found was the the Purge control solenoid was stuck wide open allowing full vacuum to the vapor line to the fuel tank..

Easy test is to remove supply vapor line to the Purge control solenoid which is usually mounted right near or intake or driver side fender, easy test is turn the key on, turn on the smoke generator and if any smoke comes out of the solenoid out of the unplugged port to fuel tank there is your problem.

Almost any scanner will allow you the test ability to perform a purge control solenoid test and it allows you to cycle it in 10% increments and should allow you to see the duty cycle while performing this test. The Purge control solenoid should be completely closed at idle and with KOEO. same test can be performed on just about any chrysler or dodge vehicles. Again very easy to find it, your computer may also show Large evap system leak.


Hope that helps


Sounds a quick easy test :+)

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