'94 Grand Cherokee 5.2L, TBI. Has OBD, not OBD-II. The Check Engine Light came on after 10-15 mins running.

Checked error codes using the ignition key method, wondering if I'm reading the flashes correctly? Why would the code 21 be repeated ?

12 | 21 | 27 | 21 | 55

Have not had chance to investigate actual problem further yet.


ps: any advice beyond check O2 sensor, injectors and PCM / injector wiring appreciated in comments. Runs fine till warm, then intermittent low power, surge, as if fuel flow drops, but not a missing. Also intermittent rough running at highway speed in OD, but switch off OD and improves.

  • 1
    Realistically, we could only guess you have read the codes correctly. If you posted a video of the flashes, we might be able to confirm you read them correctly and we could go from there. Oct 13, 2016 at 10:00
  • I corrected your 94 foot Cherokee to a 1994 Cherokee, hope you don't mind. :)
    – anonymous2
    Oct 13, 2016 at 11:56
  • I can try get/post video, but can/does a code appear twice in the sequence?
    – Ian W
    Oct 13, 2016 at 17:04
  • it shouldn't, i've never seen a code repeated in obd1. but who knows... post a video so this can be confirmed.
    – Ben
    Oct 13, 2016 at 19:12

1 Answer 1


The 21 issues has been observed by others in the past on a couple different jeep engines running OBD1. As the numbers are flashed in increasing values it is extremely unlikely it is a second 21 with it being out of place if it is an engine code.. Every reported double 21 has been a miss read to my knowledge.

It is listed in various manuals for OBD reading on the Jeep engine that there are two different 21 code meanings listed:
21**: Oxygen sensor detection neither rich or lean -or-
21**: Oxygen sensor input voltage stayed above normal operating range.

It also is known reported issue of the flashing lights have uneven spacing which causes read errors. For instance your 27 could be a 29 followed by a 65 (I am not say this is what is reading in your case). I had to have both my wife and my son read the code on a old Wagoneer taking us a dozen tries to may sure we read the lights correctly.
I would opt for borrowing or buying a reader if you can not find the timing differences.

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