I started in this whole PCM tuning thing with the objective of helping a friend get a SC application to market. One of my primary thrusts was to keep all the EPA – Smog – Federal Emissions stuff in tact so that the kit could be certified. I did not have to switch off a single on board diagnostic function to get the job done and yet all the smog bits were in place with no MIL or CELs.

This brings me to the point for starting this thread/conversation. The factory PCM has a very long list of on board diagnostics. Each has a “switch” in the firmware that allows you to change what the PCM does with the results of that diagnostic function. It goes something like turn on CEL right away, store and turn on CEL if code/error persists, do not turn on CEL but store code or just plain do not run the test at all.

Given the above, it is a simple matter to “switch off” any diagnostic function. I’ve avoided this for two reasons.

First, those diagnostic functions help me and, more importantly, the owner keep track of what is going on with the sensors and the PCM system as a whole. I know if I see a code for the purge system that I need to look at it. If there is a MAF range failure then I need to look a the MAF and so on. My primary fear of turning off any of these functions is that I simply do not know what will happen if the sensor or sensors covered by the diagnostic function fail. If there is no diagnostic to toss the car into limp home mode and protect the engine, will the failure cause the engine to take a dump? I would think that this is especially important when you are dealing with boosted applications where the engine is under more stress.

Someone mentioned Federal Law in another thread which brings me to my second reason for not turning off functions. I would think that some of those functions are used in the drive cycle tests which are mandated by the EPA. Heck, the test itself is probably mandated by the EPA. If tuning companies turn these off, or provide the capability for their tuners to turn them off, the whole enthusiast’s community is put at risk for having the tuning tools taken away. I did not think it wise to take that risk.

I am but one opinion. I’d love to hear others.

 

for some / a few codes i would keep the light off, and store the code.

anyone messing with this stuff should have a reader and check for codes often..

i would not kill any sensor codes for the same reason you give.

beers

 

oh this thread is going to need some popc0rn...


PS - For the individuals who are catless, being able to turn off the cat efficiency CEL actually lets them know if they have a problem rather then a solid never ending CEL that could mask another code popping up.

 

but things light tpms when you have switched wheels...