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Old 06-02-2010, 05:24 AM   #1
Flameeater's Avatar
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Check Engine Light

I am getting codes 12, 21, and 51 on the odometer. Any ideas?

I am thinking of changing the O2 sensors, and the fuel filter, but... I went to the local parts store and want $125 for a fuel filter!! What gives!? That is insane! I have never seen a fuel filter cost more then 5-10 bucks. Was there a mistake? When he told me that I was like

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Old 06-02-2010, 06:20 AM   #2
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Where is the fuel filter on your Jeep? My fuel filter ('98 TJ) is in the fuel tank. My Haynes Repair Manual says "Only 1987 through 1995 models use a replaceble in-line fuel filter. The manufacturer does not suggest periodic fuel filter replacement on 1997 and later models. Since the fuel filter is part of the fuel pump assembly inside the fuel tank on 1997 and later models, refer to Chapter 4 if fuel contamination makes it necessary to replace fuel filter."

I have read the referenced Chapter 4 procedure and you gotta drop the fuel tank. My B-I-L did this on his Jeep to replace the fuel pump and he's still cussing it.

My guess is the $125 is for the pump/filter assembly.

Never argue with idiots......they will drag you down to their level then beat you with experience.
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Old 06-02-2010, 08:09 AM   #3
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Fuel filter is in the's usually one of the ones that doesnt need replaced...unless you feel like dropping the tank. I think 12 & 51 are start test and end test hex can look up chrysler hex codes on the internet and proceed from there.
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Old 06-02-2010, 11:13 AM   #4
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It's a whole pump assembly that sits inside your tank and isn't one of the in-line el-cheapo ones
  • 12 - Battery Disconnect Direct battery input to the PCM was disconnected within the last 50 key-on cycles
  • 21 - Left O2 Sensor Shorted to Voltage Left oxygen sensor input voltage maintained above the normal operating range
  • 21 - Upstream O2 Sensor Shorted to Ground; or Upstream O2 Sensor Heater Failure; or Upstream O2 Sensor Slow Response Upstream O2 sensor voltage too low, tested after cold start; or Upstream oxygen sensor heating element circuit malfunction; or Upstream oxygen sensor response slower than minimum required switching frequency
  • 21 - Downstream O2 Sensor Shorted to Ground; or Downstream O2 Sensor Heater Failure; or Downstream O2 Sensor Shorted to Voltage Downstream O2 sensor voltage too low, tested after cold start; or Downstream oxygen sensor heating element circuit malfunction; or Downstream oxygen sensor input voltage maintained above the normal operating range
  • 51 - Fuel System Lean A lean air/fuel mixture has been indicated by an abnormally rich correction factor

Rather then look at the fuel filter (which are usually pretty strong), look instead at replacing the O2 sensor, and then checking your MAP and TPS sensor. They're infamous for throwing a code 51. It may have simply gotten wet. Your map sensor and TPS sensor are located right next to the throttle body. Someone else may be able to post a picture that will help tremendously.

Also, what engine do you have? The 2.5L or 4.0L?
It's got a Willy's body, a Chevy engine, Ford axles, GM transmission, and Dodge steering components... it must be a Wrangler.
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Old 06-18-2010, 06:12 PM   #5
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Hey guys,
Sorry for the looooooonng delay in getting back to you. Had personal issues come up and haven't been online. Problem with Jeep is still present. As I haven't had time to tinker with it. My TJ is a 2.5 and it seems extremely sluggish and weak. On start-up it idles very rough. Also when getting off interstate it idles at about 2k for a good while before dropping down to 1k. It is guzzling gas like no body's business. A quarter tank in 30 miles of interstate driving. The check engine light is off as of now. It comes on then goes off.

How expensive is it to replace these sensors you spoke of? Is it difficult to clean the throttle body (saw that suggestion in another thread I think).
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