Trouble codes

[rdebevoise]

ok i have a 95 xj 2wd 4.0 i have no check engine light. but i just got a new obd1 reader so to test it out i hooked it up i got 6 codes. 4 codes and 2 duplicates. i got 21 42 43 and 47 i know what the codes mean but my question is y isnt the light on? yes the light works. im thinking they are saved codes. it already has a new alt. and battery any suggestions.

[Cheromaniac]

You don't necessarily have to see the CEL come on to register fault codes in the PCM. I suggest you disconnect the battery for at least 10 minutes to reset the PCM, go for a 20+ minutes drive, and then scan the PCM again to find out if any of the previous codes reappear.

21 Oxygen sensor signal doesn't change (stays at 4.3-4.5V). Probably bad oxygen sensor
42 Automatic shutdown relay circuit open or shorted
42 Fuel pump relay control circuit
42 Fuel level unit - no change over miles
OR
42 Z1 voltage missing when autoshutdown circuit energized (SEE NOTE #6)
43 Peak primary coil current not achieved with max dwell time
OR
43 Cylinder misfire
OR
43 Problem in power module to logic module interface
47 Battery voltage too low and alternator output too low

[4.3LXJ]

Clear the codes and see what happens. If you get all those codes again instantly, then the ECM is starting to breath its last.