ECU Code 31; [Don't] Replace ECU

[MY1PATH]

ECU Code 31; [Don't] Replace ECU
I don't think this has been covered before and it may provide useful to someone someday.
If you get a code 31 which is an indicator of an ECU calculation error (how it determines timing and fuel maps) the FSM suggests you replace the ECU with new.
A fault within the ECU (shorts, worn out components, broken or corroded circuts, broken or corroded solder joints etc..) can cause this and if that it the case it is reasonable to repair or replace the ECU... BUT a code triggering fault can happen outside the ECU as well and this is what I call a Non-ECU Code 31 (a faked code)

 

So what can cause a non-ECU Code 31?

Typically wiring issues, most commonly when a 12v source crosses a sensor input or when a few sensor inputs cross ground. Old heat damaged harnesses might be prone to this as well as rodents chewing on wires but you should also look for places a harness may have chaffed and grounded out on the area chafing it.

Also, a jolt or shock to the wiring harness that specifically happened in the harness or a sensor (not inside the ECU) can cause this.
Lastly, as I have recenly discovered, trying to connect to the ecu via a conslut device or other means using the 14 pin data connector on the harness when said connector is not wired correctly.

 

What are the symptoms of a non-ECU Code 31?
Check Engine Light... Code 31 (possibly others) when you go to pull codes from the ECU.

Other Symptoms vary, it may throw multiple codes for the sensors that were shorted or it may throw multiple other codes seemingly at random. TPS (23) and Injector Circut (51) can typically be among this list but again symptoms vary.

Often when a code 31 is stored the ECU will not let you erase any of the stored codes in a fasion that you would normally clear stored codes. This is a failsafe to prevent a user from erasing the most critical code in the engine management system.

Despite all the codes your vechicle may seem too opperate 90% normal with the occasional hiccup here and there. (worse if your short is causing other problems)

How do I determine its a non-ECU Code 31 vs a legitamate code 31 and how do I fix it?

A non-ECU Code 31 (and all other codes) can be fully cleared by disconnecting the battery or unplugging the ECU for more than 24 hours.

During this time you should check all your wire harnesses for chafing, dry rotting, heat damage, rodent damage etc... If you have done an ECU retro fit, a MPFI conversion, wire harness swap or anything else similar you should get out your diagrams and double check all that too. (Check connector/ 14 pin conslult diagram is not listed clearly in the FSM I will provide more information later) If you had a Non-ECU Code 31 and you found the real electrical problem you should, after 24 hours, be able to reconnect your vehicle and resume driving like normal with no issues, no codes and no check engine light.

 

Summary

A Code 31 is not very common (real or fake) but you should always try to rule everything else out before you mistakenly throw away a good ECU.
A fake code 31 Its kinda like the ECU gets a little bug and it needs a good hard reset (24 unplugged) for it to shake it and start fresh.

[MY1PATH]

(Check connector/ 14 pin consult diagram is not listed clearly in the FSM I will provide more information later)

At this time I have no information to add on connecting a stock pathfinder ECU to any kind of diagnostic interface. The wires for this connector are the same for nearly all 90's vg30 ecu that are capable of using a diagnostic interface but these are not present in the stock pathfinder ECU connector.

[MY1PATH]

I lied, I just pulled up the MPFI pathfinder wire diagram and my Nistune ECU harness notes. The side by side images told me that the MPFI Pathfinders use the SR20 connector (Very neat and compact) instead of the 'typical' VG30 connector. But again, the diagnostic wires are not in the SR20 locations either. One day I may try to add them and see if I can connect to the ecu.

[TXPower]

Good info and write-up.

 

Thanks MY1PATH,

 

TXPower