OffcNeville

Dealing with a 2005 Civic EX that occasionally gets into limp mode (i.e. maxes out at 3k RPM). Today it's been stuck in it for a few hours. All dashboard gauges working fine. Have 3 CEL codes, most of which have been on for months:

P0340
P0134
P1456

Old forum posts for limp mode always report a mixture of partially non-functioning gauges, non-functioning CEL, unresponsive ECU (?), battery light on, engine starting problems, or random shutdowns. Usual fix is replacing the ECU, which is often fried by a poorly-grounded alternator. I have neither of those symptoms except for maybe engine startup taking a second or so longer, so I don't want to deal with the expense/tedium of the ECU when it could just be something simpler.

So, given that my only symptoms are the 3 CEL codes, anyone have any ideas as to how I should proceed? Should I address the individual CEL lights or just jump straight to the assumption of a bad ECU? I have basically zero repair experience with this car model.

ezone

Incorrect assumption. Gauges going crazy is associated with a certain PCM failure...and the PCM is really in backup mode which is effectively a type of limp.

Certain fault codes will cause the PCM to limit performance (restricted RPM limit, also called limp mode or fail safe)

The P0340 code (CMP sensor-no signal) will cause the PCM to limit power. Fix that one and your normal performance should be restored. Hell, erase the codes and performance may be normal until the code resets.



Neither of these codes will cause your performance issue:
P0134, probably needs a front AF sensor. (USE THE CORRECT DENSO SENSOR)

P1456 is a EVAP leak code/fuel tank side, could be as simple as a gas cap that doesn't seal, or could be more complicated. Basically during the system self test, the computer could not pull sufficient vacuum on the fuel tank for some reason.



HTH