"I am seeing way way way way way way to many posts here where people fling around the term limp mode. Limp mode is VERY uncommon and i have only seen it about 2 times in god knows how many years. There are in fact many redundant systems in honda ecu's that provide safety nets in case of misc sensor failures. In all honesty its redundancy is one of the beauties of the honda ecu and why many people choose to use rom editing software over true stand alones. What happens inside your ecu all boils down to which specific sensors are affected.
examples
engine coolant temp sensor - fails your car will run slightly richer and you will not have vtec, why? because the ECT sensor is used in determining fuel trims thus the reason you would be running slightly richer. its a safety protocol built in to the ecu in the case of an ECT failure. and obviously vtec will not work since you must have a certain operating temp established in order for the system to be live. oh and a constant cold rev limit in some cases
knock sensor - a damaged knock sensor will not put you in limp mode. it will in fact retard your timing as a safety protocol and check back every couple revolutions to see if the problem has been corrected. same holds true if a correctly functioning knock sensor is detecting knock
o2 sensor (primary)- will not put you in limp mode. when the primary o2 fails your ecu is forced into open loop mode and will rely on the ecu's default maps that came from the factory or what would be the equal of a reset. during this time no long term or short term fuel trims will be done as there is nothing providing input to alter the trims, thus you will just run overly rich like you would on a fresh reset.
o2 sensor (secondary)- no limp mode here either, hell this one cause next to no side effect. its basically an expensive way to make sure your CAT is functioning withing federal specs.
iat sensor - will not cause limp mode, it will just ever so slightly alter your fuel trims towards the richer side to compensate for its failure
iacv (idle air control valve)- limp mode = the negatory on this one. you will just have a shitty idle
what does cause limp mode? like the real one true limp mode.... core sensor failure...distributor failures like the cyp taking a shit or loosing the rpm signal, map failure, catastrophic ecu circuitry failures that are internal. even this has a wide margin of outcomes. a map failure will cause the car to run like absolute shit but still somewhat ok. on the other hand a primary injector circuit failure will cause the worst limp mode of them all in which you will find getting up to 20mph if as difficult as gary coleman trying to slam dunk without a trampoline.
now this isnt the most technical thing ever but hopefully it helps clear up some peoples misconceptions about what limp mode is and when it comes into play and also about hondas redundant systems and failsafes for non critical sensor failures."
_Rick V at Superhonda
