Here's what the manual says.
The PCM's internal circuitry can identify if the vehicle fuel system is capable of supplying adequate amounts of fuel during heavy acceleration (power enrichment). When a power enrichment mode of operation is requested during Closed Loop operation, the PCM will provide more fuel to the engine. Under these conditions, the PCM should detect a rich condition.
DTC 55 will set when Engine is operating in Closed Loop,
not in fuel cutoff, (no DTCs 44, 45, 64, 65 set). If the PCM detects a lean oxygen sensor voltage for 3 seconds during power enrichment modes of operation, DTC 55 will set.
Attach a fuel pressure tester to the fuel rail. With the engine running, the pressure should be around 41-47 psi.
Turn the engine off. The fuel pressure should VERY slowly bleed off.
Some of the causes could be a bad fuel pump or clogged strainer or clogged fuel filter.
