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Engine light came on. Mechanic says oxygen sensors or catylitic converter. Shouldn't it give a specific code to determine which?

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  • cheap repair Oct 08, 2010

    first check for vacuum leaks, broken vacuum elbows, hoses, gaskets, etc



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What code did your mechanic pull from the cars computer? If it was a cat inefficiency code that usualy means the cat is going bad. If it is a sensor 2 high voltage or rich limit than it could be either. If its sensor 2 slow to respond it might be either.

If you could post the specific code/codes that would help a lot to answer your question.

Posted on Oct 08, 2010

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Yes. the cat converter codes are normally po420 or po430. o2 sensor codes vary greatly but the main one are po171 or po172[engine running lean or rich]. their are a few other codes for the heater circuits in the o2 sensors also.

Posted on Oct 08, 2010

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Yes, I would say that on an OBD-II system on a car 1996 and up it should be able to detect which Oxygen sensor is faulty or giving a false signal, or with a little bit of diagnostic time determine if the catalytic converter is plugged or not functioning using a sniffer.

Posted on Oct 08, 2010

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Hello! My long time (recently not so helpful) mechanic told me my engine light was on due to an O2 sensor code. This lasted at least a year and now the car is giving a code for the Cat Converter. Do you...

replace the o2 sensor,, clear the codes ,, and see if that cat code comes back, they work in unison,, so you may just need the O2 sensor, and all codes will clear

Aug 31, 2011 | 2003 Toyota Corolla

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I have 6 cyl 03 ford ranger, check engine light came on and had it read and code says po422 EVAP, any help would be appreciated

The catalytic converter is no longer functioning properly
An oxygen sensor is not reading (functioning) properly
There is an exhaust leak

Sometimes simply resetting the code is all you need to do. Many codes will reset themselves over time. What the person meant was that in normal driving the code may reset itself. You don't need to waste your time or fuel driving nowhere to attempt to reset the code.

Personally, here is what I would do if I were you. If the place in Texas where you are moving to does not require emissions testing, do nothing for now. If the car seems to be running normally the code may reset itself. If Texas requires emissions testing you will need to have the problem addressed or the car will not pass emissions. Bring the car to a reputable independent mechanic for repairs. If it is your catylitic converter expect some sticker shock as these can be expensive. Hope this helps, Please rate this!!! :) James Booth

May 23, 2011 | 2003 Ford Ranger Regular Cab

2 Answers

I have a dodge ram 2500 5.9 cummins showing a code of p0140. What's this code

From this website:

P0140 - 02 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)

What does that mean?

The Powertrain control module (PCM) will provide a .45 volt reference voltage to the Oxygen sensor. When the O2 sensor reaches operating temperature, it will generate a voltage that will vary depending on the oxygen content of the exhaust. Lean exhaust generates a low voltage (less than .45V) and rich exhaust generates a high voltage (greater than .45V). O2 sensors on a specific bank marked as "sensor 2" (as this one is) are used to monitor emissions. A Three-Way Catalyst (TWC) system (catalytic converter) is used to control tailpipe emissions. The PCM uses the signal received from Oxygen sensor 2 (#2 indicates aft of catalytic converter, #1 indicates pre-converter) to read efficiency of TWC. Normally this sensor will switch between high and low voltage at a noticeably slower rate than the front sensor. This is normal. If the signal received from rear (#2) O2 sensor indicates that the voltage has "stuck" between .425V to .474 V, the PCM determines this sensor is inactive and this code will set.

Potential Symptoms

Your check engine light(CEL), or malfuction indicator lamp (MIL) will be illuminated. There will not likely be any noticeable drivability problems other than the MIL. The reason is this: The rear or post catalytic converter Oxygen sensor does not affect fuel deliver(this is an exception on Chryslers). It only MONITORS the efficiency of the catalytic converter. For this reason, you will likely not notice any engine trouble.


The causes for a P0140 code are fairly few. They could be any of the following:

Shorted heater circuit in O2 sensor. (Usually requires replacement of heater circuit fuse in fuse block also)

Shorted signal circuit in O2 sensor

Melting of harness connector or wiring due to contact with exhaust system

Water intrusion in harness connector or PCM connector


Possible Solutions

This is a fairly specific problem and shouldn't be too difficult to diagnose.

First, start engine and warm up. Using a scan tool, watch the Bank 1, sensor 2, o2 sensor voltage. Normally the voltage should switch slowly above and below .45 volts. If it does, the problem is likely intermittent. You'll have to wait for the problem to surface before you can accurately diagnose.

However, if it doesn

Apr 15, 2011 | Dodge Pickup Cars & Trucks

1 Answer

The Egine Lights of my Kia Sportage 1999 is always on when i am driving and it wont turn off. Some mechanic recommend that i must replace the cat convertor and oxygen sensor can that be true before i buy...


When the check engine lights comes on there are codes stored in the ECU module (car main computer.
To know what must be replaced you need to find out what codes are stored in the ECU.
Post 1995 codes adhere to OBDII standard (see here: Kia OBD-II Trouble Codes). The codes can be retrieved connecting the ECU to a code scanner (like this:Actron CP9575), and performing a scan.
If you live in the US, code scanning can be done for free at the nearest Autozone, otherwise you can have a code scanning done at your garage for a small fee.

Regarding the mechanic's suggestion of replacing the cat convertor and oxygen sensor, that is probably based on testing. If the mechanic has checked the emissions, and emissions are hight though the engine is OK, then it may time to replace the cat converter. The mechanic can also decide to replace the cat converter if the part is not in good shape.

If the decision is based on an O2 sensor OBDII code,before replacing the sensor better having a quick look at emission and engine.

The best way to proceed is generally determined by the code returned and by the checkup of state of the engine, O2 sensor, and exhaust system.



Jan 07, 2011 | 1999 Kia Sportage

1 Answer

I have a 2003 tacoma pick up truck and the check engine light went on. I took it to my mechanic and he said the p0420 code came up and it was a catalytic converter. I only have 38,000 miles on it so he...

Well, the P0420 means "Catalyst System Efficiency Below Threshold (Bank 1)" - that both oxygen sensors (the one before the converter and the one after the converter) are reading about the same, meaning the converter is not working within the set efficiency envelope.

the "Bank 1" is just a way to differentiate between right and left for dual exhaust and dual catalytic converters - Bank 1 or Bank 2

Then they have (Bank 1 Sensor 1) (Bank 1 Sensor 2) (Bank 2 Sensor 1) (Bank 2 Sensor 2)

In any event, it sounds like Toyota was correct, but, WOW , A few hundred? Considering the Oxygen Sensors cost between $124 and $194 (front or rear) - you can safely say the part will cost $200 - even at the local parts house. Then the shop labor - which will NOT take more than 20 mins to replace since they nailed which one needs replacing. Unless its a ****** doing it. They have shop minimums though I'm sure.

Good Luck and Take Care

Oct 08, 2010 | Toyota Tacoma Cars & Trucks

1 Answer

2001 buick regal w/ bad ignition coil and clogged catylitic conv.

Hello! The defective ignition coil more than likely contaminated the downstream O2 sensor...If there isn't a trouble code and engine performs in a normal fashion, there's no reason to change the upstream sensor...


Aug 03, 2010 | 2001 Buick Regal

1 Answer

Jag S-type, engine light came on. Took it to a

The best thing to do is go to an exhaust specialist, who can check the catylitic converter and also the oxygen sensors. You will need to replace these items if they are at fault. Hope this helps

Nov 16, 2009 | 1995 Jaguar XJ6

1 Answer

Catalytic converter or oxygen sensors?

i would replace the oxygen sensor first as it is the cheapest and easiest

Nov 06, 2009 | 2004 Hyundai Santa Fe

1 Answer

Catylitic Converter is not fully consuming unspent fuel

 This is an expensive repair...catalytic coverters need replacing if everything else checks out on the car.  However, a loose oxygen sensor lead would make the car running rich

Mar 27, 2009 | 2002 Lincoln LS

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