Ah, yes!  The infamous 1997 TJ Check Engine Light; official designation, the Malfunction Indicator Lamp (MIL).  If you have a 1997 TJ, then you have most likely had this stupid light come on one time or another.  What does it mean?  What do you do when it comes on?  And, most importantly, how do you turn the blasted thing OFF ?!?!?

Keep in mind, this guide only applies to the 1997 Wrangler.  It DOES NOT apply to 1996 or 1998 thru 2006 TJ, nor 2007 thru current JK and JKL models.

The Powertrain Control Module (PCM) (a.k.a., the computer or "black box") pictured below, is mounted to the firewall, under the hood, just above the battery.  Its job is to monitor numerous different circuits in the fuel injection, ignition, emission, and engine systems.  If the PCM senses a problem with a monitored circuit often enough to indicate an actual problem, it stores a Diagnostic Trouble Code (DTC) (a.k.a., an Error Code), in the PCM's memory.  Diagnostic trouble codes that affect vehicle emissions will cause the Malfunction Indicator Lamp (MIL) (a.k.a., the Check Engine Light) to illuminate continuously while the engine is running.  If the problem is repaired or ceases to exist, the PCM cancels the code after 40 warm-up cycles and the Malfunction Indicator (check engine) Lamp will go out.

Note:  Diagnostic Trouble Codes are the results of a system or circuit failure, but do not directly identify the failed component or components.


A service technician can display a Diagnostic Trouble Code (DTC) in three different ways (you and I can use two of these methods, unless you have access to a "DRB Scan Tool"; I don't):

   1. a two-digit number flashed on the Malfunction Indicator (check engine) Lamp
   2. a two-digit number displayed on the vehicle odometer
   3. using a DRB Scan Tool, plugged into the 16-way Data Link (diagnostic) Connector under the dash 
       on the driver's side


To determine which Diagnostic Trouble Code(s) are stored in the PCM of your '97 TJ:    

• Turn the ignition key to the "On" position three times in succession (i.e., On, Off, On, Off, On), within five seconds    

• The Malfunction Indicator (Check Engine) Lamp will flash the appropriate number of times to indicate the DTC number, with a short pause between the first and second digits  (e.g., 2 flashes of the MIL, followed by a short pause, followed by 5 more flashes, indicates error code 25)

• The appropriate DTC will also be displayed in the odometer window

Note: All DTCs consist of 2 digits (see table below).  If more than one DTC is stored in the PCM's memory, the procedure above will display all codes in succession.  Regardless of how many codes are stored, Code 55 will always be the final code to be displayed.  Code 55 indicates the "completion of the fault code display on the Check Engine lamp".

How to turn OFF the Check Engine Light

First of all, your Check Engine Light probably came on for a good reason.  However, if you were doing something funky on the trail, the problem has most likely already corrected itself as soon as you got things back under control.  Therefore, as stated in the Powertrain Control Module discussion above, "if the problem is repaired or ceases to exist, the PCM cancels the code after 40 warm-up cycles and the Malfunction Indicator (check engine) Lamp will go out."  This means that, depending on how many times you stop and restart the engine during the remainder of the ride, the light may very well go out before you hit the trail end.  Otherwise, it will go out during the next few days (or maybe weeks).  To get it to do out sooner, I'm told that if you disconnect the battery for a few minutes, that will reset the computer and turn off the light.  However, if there's still a valid malfunction, the light will most likely come back on again, and you should probably take your Jeep in for a professional diagnosis and service.

Diagnostic Trouble (Error) Code Descriptions:

11*     No crank reference signal detected during engine cranking.
11**    Intermittent loss of either camshaft or crankshaft position sensor. CKP sensor target windows    
            have too much variation.
12*     Direct battery input to PCM was disconnected within the last 50 Key-on cycles.
13**    No difference recognized between the engine MAP reading and the barometric (atmosphere)      
            pressure reading from start-up.
14**    MAP sensor input above or below acceptable voltage. 5 volt output to MAP sensor open.
15**    No vehicle speed sensor signal detected during road load conditions.
17**    Engine did not reach operating temperature within acceptable limits. Engine does not reach
            20º F. within 5 minutes with a vehicle speed signal.
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.
22**    Engine coolant temperature sensor above or below acceptable voltage.
23**    Intake air temperature sensor input above or below acceptable voltage.
24**    Throttle position sensor input above or below acceptable voltage. TPS signal does not correlate
            to MAP sensor.
25**    A shorted or open condition detected in one or more of the idle air control motor circuits. Actual
            idle speed does not equal target idle speed.
27**    Injector #3, and/or 4, and/or 5, and/or 6 output driver does not respond properly to the control
            signal.
31**    An open or shorted condition detected in the A/C clutch relay circuit. Insufficient or excessive
            vapor flow detected during evaporative emission system operation.
33*      An open or shorted condition detected in the duty cycle purge solenoid circuit.
34*      An open or shorted condition detected in the Speed Control vacuum or vent solenoid circuits.
            Speed control switch input below the minimum acceptable voltage.
37**    Relationship between engine speed and vehicle speed indicates no torque converter clutch
            engagement. An open or shorted condition detected in the torque converter part throttle unlock
            solenoid control circuit. Incorrect input state detected for the Park/Neutral switch. Auto only.
41***    An open or shorted condition in the generator field control circuit.
42*       An open or shorted condition detected in the auto shutdown relay circuit. An open condition
            detected in the ASD relay output circuit. An open or shorted condition detected in the fuel pump
             relay control circuit. An open circuit between PCM and fuel gauge sending unit. Circuit shorted to
            voltage between PCM and fuel gauge sending unit. No movement of fuel level sender detected.
43*      Peak primary circuit current not achieved with maximum dwell time
43**    Misfire detected in one or more cylinders 1 thru 6. (4 and 6 cyls.)
44**    Battery temperature sensor in voltage above or below acceptable range.
46***   Battery voltage sense input above target charging voltage during engine operation.
47***   Battery voltage sense input below target charging during engine operation. Also, no significant
            change detected in battery voltage during active test of generator output circuit.
51**    A lean air/fuel mixture has been indicated by an abnormally rich correction factor.
52**    A rich air/fuel mixture has been indicated by an abnormally rich correction factor.
53**    PCM Internal fault condition detected.
54**    No camshaft signal detected during engine cranking.
55*     Completion of fault code display on Check Engine lamp.
63*     Unsuccessful attempt to write to an EEPROM location by the PCM.
64**    Catalyst efficiency below required level. (Same as code 72)
65*     Power steering high pressure seen at high speed. (2.5L only)
72**    Catalyst efficiency below required level. (Same as code 64)
77*     Malfunction detected with poser feed to speed control servo solenoids.

* = This DTC will not cause the Check Engine Lamp (MIL) to illuminate.  However, if another problem causes the MIL to illuminate, and this code is identified when you display all of the codes, at least you will know what it means.

** = The Check Engine Lamp (MIL) will illuminate during engine operation if this DTC is recorded.

*** = Generator Lamp will also be illuminated if this DTC is recorded.

Source: 1997 Jeep Wrangler Factory Service Manual; Group 25, Emission Control Systems.