replaced O2 sensor, error code still there! - JeepForum.com
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post #1 of 18 Old 12-22-2007, 12:06 AM Thread Starter
fish4life
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replaced O2 sensor, error code still there!

So i replaced my upstream oxygen sensor 3 days ago only to get the code back the day after. The code i get is 21.

If everything was done right should the code go away on it's own, or does it have to be manually cleared, if so how?


Thank you

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post #2 of 18 Old 12-22-2007, 12:32 AM
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it will go away on its own after so many complete cycles that it doesnt detect the code, but it can take awhile. have the code cleared (or buy a cheap code reader then can clear codes) and see if it ever comes back


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post #3 of 18 Old 12-22-2007, 12:46 AM
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Disconnect the battery and let it sit for a few. Then reconnect the battery. It should reset that code. if it comes back on then there is still a problem somewhere.

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post #4 of 18 Old 12-22-2007, 01:49 AM Thread Starter
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Quote:
Originally Posted by Lcars
Disconnect the battery and let it sit for a few. Then reconnect the battery. It should reset that code. if it comes back on then there is still a problem somewhere.
As i was replacing the O2 sensor and radiator that day the first thing i did was disconnect the battery. After all was done i reconnected it, about 3 hrs later i think and i got no code other then 12, which is obvious.

I went off roading the next day and mid-way through my trail the "check engine" lamp came on. I cycled the on-off and got the damn 21 code back

Anyone know what i should do next? I guess there must be something else causing this problem?

FYI, the codes i get are 12, 21 then 55. So the jeep isn't telling me much ... suggestions?
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post #5 of 18 Old 12-22-2007, 01:54 AM
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Pull the connector apart and have a look if all the pins are lined up properly. I replaced mine years ago and one of the pins got pushed inside the connector stopping it connecting.
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post #6 of 18 Old 12-22-2007, 02:51 AM Thread Starter
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I did that. All connections seem solid and the pins aren't rusted or anything like that ...
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post #7 of 18 Old 12-22-2007, 07:49 AM
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You may have to check the harness wiring from the Powertrain Control Module (PCM) all the way out to the sensor connector using a multi-meter.

Anyway you can get to a code reader to get the OBD-II code value? This will give more information. Autozone, if local to you, will extract the code for free.

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post #8 of 18 Old 12-22-2007, 07:54 AM
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Quote:
Originally Posted by Slithering_Joe
You may have to check the harness wiring from the Powertrain Control Module (PCM) all the way out to the sensor connector using a multi-meter.

Anyway you can get to a code reader to get the OBD-II code value? This will give more information. Autozone, if local to you, will extract the code for free.
21 Upstream oxygen sensor response slower than minimum required switching frequency. Upstream oxygen sensor heating element circuit malfunction. Downstream oxygen sensor heating element circuit malfunction. Downstream oxygen sensor input voltage maintained above the normal operating range. Oxygen sensor voltage too low, tested after cold start. (Upstream or Downstream) Left oxygen sensor input voltage maintained above the normal operating temperature.

John 3:17
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post #9 of 18 Old 12-22-2007, 08:20 AM
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Dogman, I have read that from another site but that description refers to multiple sensors. "21" is a Chrysler internal code. OBD-II codes are more specific and may narrow the problem down.
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post #10 of 18 Old 12-22-2007, 01:16 PM Thread Starter
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Quote:
Originally Posted by Slithering_Joe
You may have to check the harness wiring from the Powertrain Control Module (PCM) all the way out to the sensor connector using a multi-meter.

Anyway you can get to a code reader to get the OBD-II code value? This will give more information. Autozone, if local to you, will extract the code for free.
hmmm my father has a ODB II reader for his Ford, though i didn't think 1997 Jeeps had the connections for ODB II .... but now that i think about it is it the little connector underneath the steering wheel???

I will have to give that a try and report back with more info. However, can you elaborate more on checking the PCM connections with the multi meter. What exactly am i checking for? Just that the connections are solid, or for a specific resistance in ohms or what?

Thanks so much
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post #11 of 18 Old 12-22-2007, 02:33 PM
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Quote:
Originally Posted by fish4life
hmmm my father has a ODB II reader for his Ford, though i didn't think 1997 Jeeps had the connections for ODB II .... but now that i think about it is it the little connector underneath the steering wheel???
...
All 1996 model year vehicles sold in the US are federally mandated to support OBD-II. Some vehicles had supported OBD-II starting with 1994 model year. In fact, Honda and Toyota were fined because some models did not support OBD-II.

A generic OBD-II code reader that worked on your father's ODB-II Ford will work on any OBD-II vehicle including your Jeep. I would start with extracting the Powertrain diagnostic trouble codes P0XXX (0 = generic) or P1XXX (1 = manufacturer specific). Trouble codes involving O2 sensors can be tricky. Many assume that the senor is bad but the problem maybe a vacuum leak or a stuck/clogged/leaking fuel injector. Getting this "P" code would be a great start.

Quote:
Originally Posted by fish4life
...However, can you elaborate more on checking the PCM connections with the multi meter. What exactly am i checking for? Just that the connections are solid, or for a specific resistance in ohms or what?...
Installing a new sensor will not fix your problem if you are unlucky to have a wiring harness issue. Do a simple visual inspection of the wiring harness to see if any wires are damaged. If that checks out, you may need to see if there is high resistance in the wiring between the O2 sensor connector and the PCM connector. There are two circuits to an O2 sensor - the heater circuit and the sensor circuit. High resistance may be caused by a break in the wire (up to infinite resistance) or corrosion (~ greater than 10 ohm). Obviously, a good wire will have very low resistance. To check this, start by disconnecting the battery, disconnect the O2 sensor from the vehicle's factory harness and disconnect the PCM from the vehicle's harness. At this point you may want to inspect the connector to see if the pins are seating correctly. Next, using a digital multi-meter set to continuity/resistance, you will need to probe both side of the harness (O2 Sensor side and PCM side) to get a resistance reading. DO NOT probe the PCM and DO NOT probe the O2 sensor. Damage may result. Below is the wiring outline for the two O2 sensors.

Upstream O2 Sensor (from 1998 Service Manual, assuming '97 is the same)
Orange/Dark Green: (+)12V Sensor Heater from 20A Fuse 21 in the engine compartment fuse relay box
Black: (-)Heater circuit ground
Black/Dark Green: Sensor Signal to pin A24 on PCM
Brown/Yellow: Sensor return to pin A4 on PCM

Downstream O2 Sensor (from 1998 Service Manual, assuming '97 is the same)
Orange/Dark Green: (+)12V Sensor Heater from 20A Fuse 21 in the engine compartment fuse relay box
Black: (-)Heater circuit ground
Tan/White: Sensor Signal to pin A25 on PCM
Brown/Yellow: Sensor return to pin A4 on PCM

1998MY TJ PCM Connector C2


1998MY TJ Downstream O2 Sensor Connector: (Upstream O2 Sensor similar, gray)


Good Luck.

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Last edited by Slithering_Joe; 12-22-2007 at 02:50 PM.
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post #12 of 18 Old 12-22-2007, 04:51 PM Thread Starter
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Just a couple more questions regarding this interesting problem....

My internet search has not been very conclusive on whether the "Bank 1 sensor 2" is the oxygen sensor found near the Catalytic converter or near the exhaust manifold.
The reason i ask is that the Jeep tells me the error code is "21" which means "upstream O2 sensor" when i do the ON-OFF sequence. But when i connect the OBD Reader it tells me P0138 which sounds like the downstream sensor .... and i doubt both would be shot at the same time, seems very unlikely.

Also ... the Catalytic converter has been replaced several months ago, could that in itself cause a code?

Slithering Joe, thanks for that detailed how-to. I will do that as soon as the rain dies down outside. Thanks very much
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post #13 of 18 Old 12-22-2007, 05:41 PM
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according to all data it is a downstream o2 sensor shorted to voltage. the possible causes is gives are;shoeted output circuit, bad sensor, pcm, connector, and open circuit. hope this helps!
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post #14 of 18 Old 12-22-2007, 09:23 PM
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Quote:
Originally Posted by fish4life
Just a couple more questions regarding this interesting problem....

My internet search has not been very conclusive on whether the "Bank 1 sensor 2" is the oxygen sensor found near the Catalytic converter or near the exhaust manifold.
Bank 1 is the side of the engine that contains the #1 cylinder. V8 & V6 engines have 2 banks. Bank naming on inline-6 engines vary between manufacturers. Typically an inline-6 only has one bank but there are a few manufacturers that treat the first 3 cylinders (#1, #2 #3) as bank 1 and the last 3 cylinders (#4, #5 #6) as bank 2. This is based on the exhaust manifold configuration.
Sensor 1 is before the catalytic converter.
Sensor 2 is after the catalytic converter.


Quote:
Originally Posted by fish4life
The reason i ask is that the Jeep tells me the error code is "21" which means "upstream O2 sensor" when i do the ON-OFF sequence. But when i connect the OBD Reader it tells me P0138 which sounds like the downstream sensor .... and i doubt both would be shot at the same time, seems very unlikely.
I cannot find a solid description on code "21." Some say it is the upstream sensor problem and while others say it is downstream sensor problem. Even look at post #8 above. I found that description too but it is all over the place!
You really can't assume that both sensors are shot because you haven't proved that the upstream sensor was bad! And from the sound of it a code 21 deals with the downstream sensor since you extracted a P0138 using the OBD-II interface. Therefore, the upstream sensor is unrelated. Understand?
My guess, since this is hard to do over the internet, there may be an open circuit somewhere in the sensor signal wiring or the O2 sensor connector to harness connector connection. If that is OK, then the sensor is bad. (Worst case scenario after you have exhausted every possible diagnostic test, the PCM is bad. This is rarely the case.)

Here is some good reading on O2 sensor operation: http://www.aa1car.com/library/o2sensor.htm


Quote:
Originally Posted by fish4life
Also ... the Catalytic converter has been replaced several months ago, could that in itself cause a code?
Aftermarket (not factory) catalytic converters tend not to be as efficient as the factory OE (original Equipment). Aftermarket makes the absolute minimum compared to the factory. If a cat is functioning below efficiency, you will get a P0420 code (Catalyst System Efficiency Below Threshold [Bank 1]) If the sensor was damaged during the instllation of the sensor then that is a possibility.
Additional reading if you like: http://www.aa1car.com/library/p0420_dtc.htm


Quote:
Originally Posted by fish4life
Slithering Joe, thanks for that detailed how-to. I will do that as soon as the rain dies down outside. Thanks very much
No problem.

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Last edited by Slithering_Joe; 12-22-2007 at 10:09 PM.
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post #15 of 18 Old 12-22-2007, 11:37 PM
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Does it have aftermarket exhaust? Sometimes headers wont let the O2 sensor heat up to get an accurate reading. I am not sure if yours is using a heated O2 sensor or not.

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