Engine codes 51 & 21?(help again!) - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 06-22-2003, 09:42 PM Thread Starter
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Engine codes 51 & 21?(help again!)

I just checked the codes on my 4.0 and it came up with 51 and 21.

51** Oxygen sensor signal input indicates lean air/fuel ratio condition during engine operation.
21** Neither rich nor lean condition detected from the oxygen sensor input.
or
Oxygen sensor input voltage maintained above the normal operating range.

Does this mean my o2 sensor is bad? I just replaced it about a year ago or so. Maybe a little longer than that. I did run the Jeep with no exhaust on it for about 15 miles or so when I took it to the muffler shop. Could this of damaged it? What should I check? Thanks.
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post #2 of (permalink) Old 06-22-2003, 10:04 PM
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Re: Engine codes 51 & 21?

This Reading Computer Codes page says>>

21-Oxygen sensor or circuit.=O2 sensor or wiring faulty.
51-O2 detects lean mixture.=Check sensor and circuit and for vacuum leaks.

Have you tried disconnecting the battery over night and see if that helps? [img]images/graemlins/confused.gif[/img]
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post #3 of (permalink) Old 06-24-2003, 12:53 AM Thread Starter
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Re: Engine codes 51 & 21?

I disconnected my battery tonight for about an hour. I also plugged a vacuum line that led to nothing. So basically it was a leak. I've driven about 20 miles or so so far with out getting any codes. Is it possible that small vacuum leak could of caused this problem? The first few miles I drove the Jeep it idled real low, but seems better. I assume it's just the EFI adjusting to the driving conditions? Think I'm in the clear, or think it'll take a bit longer for the codes to come back?
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post #4 of (permalink) Old 06-29-2003, 11:50 PM Thread Starter
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Re: Engine codes 51 & 21?

After about 200 miles or so, I checked my codes again. I don't have a code 21 anymore, but the 51 is back. What else can I do/check? Should I try replacing the O2 sensor? This is a horrible time for this to happen, as I have a trip coming up in almost a week. Any help ASAP would be greatly appreciated.
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post #5 of (permalink) Old 06-29-2003, 11:54 PM
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Re: Engine codes 51 & 21?

What made you check the codes, did the check engine light come on again?
John
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post #6 of (permalink) Old 06-30-2003, 12:32 PM Thread Starter
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Re: Engine codes 51 & 21?

No, no check engine light. I just decided to check them about a month ago when I first got my Jeep running again after 6-7 months. That's when I first noticed it. Well, I unhooked my battery and plugged my only vacuum leak that I know of. It seemed to fix the problem, but now code 51 is back. One thing I was thinking about was that I know I have a small exhaust leak before the o2 sensor, could this be doing it? Also, what damage will I do if I take my Jeep on a roadtrip to Naches, WA? It's about 5 hours one way.
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post #7 of (permalink) Old 06-30-2003, 05:21 PM Thread Starter
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Re: Engine codes 51 & 21?

BTT - Anyone have any ideas?
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post #8 of (permalink) Old 07-01-2003, 01:40 PM Thread Starter
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Re: Engine codes 51 & 21?

BTT one last time. Nobody has any ideas at all? [img]images/graemlins/thud.gif[/img]
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post #9 of (permalink) Old 07-01-2003, 01:53 PM
 
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Re: Engine codes 51 & 21?

</font><blockquote><font class="small">In reply to:</font><hr />
One thing I was thinking about was that I know I have a small exhaust leak before the o2 sensor, could this be doing it?

[/ QUOTE ]

It's very possable because a leak would let oxygen in and be thowing off the reading. I'd fix the leak before I did anything else. Did you check the O2 with a DVM to see what the voltage readings are?

</font><blockquote><font class="small">In reply to:</font><hr />
Also, what damage will I do if I take my Jeep on a roadtrip to Naches, WA? It's about 5 hours one way.

[/ QUOTE ]

I doubt it would do any damage since it doesn't seem to be a hard code, even if the O2 fails the computer has limp home mode built into it.
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post #10 of (permalink) Old 07-01-2003, 02:29 PM
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Re: Engine codes 51 & 21?

It might just be an old code I'd try and fix the exhaust leak and disconnect the battery for atleast 5-10 min. Take it out for a spin and see if the code returns.
John
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