Limp Home Mode was created by GM and used in OBD-1 computers. It was designed to put the computer into a "limp-home" mode should any one of the following conditions exist:

• ECM input voltage is below 9 volts.
• Cranking voltage is below 9 volts.
• PROM missing or not functioning correctly.
• ECM internal circuitry fails to ensure proper Computer Operating Pulses.

When any of the above conditions existed, the OBD-1 ECM would revert to a basic operating state and run off the cal-pak (back-up fuel) chips which were seperate from the main PROM chip (which contained the calibration). Around 1994 model year, GM started gearing up for OBD-2. And much of what the limp home mode did in earlier computers was dropped. OBD-2 computers still have what I guess you could call limp modes, but GM now refers to it as "the action the PCM takes" when a problem is detected.

In OBD-2 PCMs, certain trouble codes being set will cause the PCM to take certain action to prevent damage or allow the car to still safely operate. For example, if the PCM detects a transmission issue and sets a transmission DTC, it may command a transmission "limp mode" -- which may limit the gears the transmission operates in; or the amount of line pressure to be fixed; or prevent TCC lockup from operating. Most transmission "limp-modes" won't affect engine operation at all. But some may. Depends on the application and the specific DTC set. Same thing goes for the engine.

Certain DTC's have "limp mode" parameters attached to them which are implemented when they set. And there are several different actions that the PCM can be take (aka: limp modes that can be implemented), depending on the nature of the problem. All limp-modes that exist in OBD-2 PCMs are tied directly to certain DTC's. If you are curious about certain limp-modes, you should look at the DTC breakdown in the GM service manual for an explaination of what happens when certain DTC's set ("action taken by PCM when DTC sets"). Obviously below a certain internal/battery operating voltage or catastrophic failure, the PCM circuitry will just shut down and not work at all.

I'll give you some brief examples for a 1998 GTP...

P0322 Knock Sensor Circuit Bank 2 Error: ACTION TAKEN WHEN THE DTC SETS
^ The PCM will not illuminate the Malfunction Indicator Lamp (MIL).
^ The PCM will store conditions which were present when the DTC set as Fail Records data only. This information will not be stored as Freeze
Frame data.
^ The PCM will use a calculated spark retard value to minimize knock during conditions when knock is likely to occur. The calculated value will
vary based on engine speed and load. <-- this could be called an engine limp mode

P0716 Input Speed Sensor CKT Range/Performance: ACTION TAKEN WHEN THE DTC SETS
^ The PCM illuminates the Malfunction Indicator Lamp (MIL).
^ The PCM disables shift adapts.
^ The PCM inhibits Torque Converter Clutch.
^ The PCM inhibits 4th gear when the transmission is in Hot Mode
^^^the above could be considered to be a transmission limp mode

Below is an example of a problem with the engine that will cause a transmission limp mode...

P0300 Engine Misfire Detected: ACTION TAKEN WHEN THE DTC SETS
^ If the misfire level is non-catalyst damaging, the PCM will illuminate the MIL during the second key cycle in which the DTC sets.
^ If the misfire is severe enough to cause possible catalyst damage, the PCM will immediately flash the MIL while the misfire remains at catalyst
damaging levels.
^ The PCM will disable TCC operation.
^ The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data.

Some DTC's won't make the PCM take any action whatsoever (aka: implement any kind of limp mode), so keep that in mind as well. It is also worth mentioning that not every action the PCM may take when a certain code sets is mentioned in the service manual. GM wrote the service manual for the general tech in the repair shop to just follow a diagnostic flow chart if something was discovered to be wrong. I guess they didn't want to confuse people with too much information; rather they just wanted them to replace part A if test B showed "no".

-ryan