Code 55, 5 Volt Reference Circuit
The ECM regulates ignition supplied voltage from ECM terminal "A" down to 5 volts, which it supplies to MAP Sensor and Metering Valve Sensor (MVS) through ECM terminal "C". To aid in diagnosis, a pull-up resistor is connected between 5 volt reference terminal "C" and MVS terminal "R" inside ECM. If MVS circuit from ECM terminal "R" is open, voltage reading at terminal "R" will be about 5 volts. Code 55 indicates ECM reads voltage that is too high or too low at ECM terminal "C".
NOTE:
The following step numbers refer to the numbers in the accompanying flow chart(s).
- Checks ECM terminal "C" voltage with ignition on and engine stopped. Voltage should read 4-6 volts.
- Determines if high voltage is result of short to battery voltage to or faulty ECM.
- Determines if low voltage is due to grounded circuit, MAP sensor, MVS, or faulty ECM.
- Since voltage reading in step 1) was normal, intermittent short or ground in ECM terminal "C" circuits may be at fault.