The powertrain control module (PCM) uses information from the intake air temperature (IAT) and the engine coolant temperature (ECT) sensors to determine ambient temperature at cold startup. Alcohol content, alcohol percentage, information is supplied to the PCM from the fuel composition sensor (FCS). When the alcohol content is too high for a given ambient temperature an internal diagnostic trouble code (DTC) will set in the PCM. This code is for information only, in order to aid the technician for symptom related complaints such as hard starting in cold ambient temperatures due to fuel blends which are not recommended for the ambient temperature. In some cases the DTC may set if there is contamination present in the fuel causing the calculated alcohol content to be higher than expected.

The ECT and IAT have less than 4.5°C (8°F) difference at cold start up.

The PCM monitors the fuel composition frequency during the first 30 seconds of run time. If the fuel composition is above the given threshold for the ambient temperature, for 25 seconds of the 30 second interval, DTC P0169 will set. The table for temperature versus frequency thresholds is contained in Diagnostic Aids.

The system is designed for use of regular unleaded fuel to any ethanol concentration up to 85 percent (E85). When the ambient temperature drops below minus 10°C (14°F), a winter blend of 70 percent (E70), or less is recommended for cold weather starting. If the ethanol concentration is higher (E85), a DTC P0169 will set. Adding regular unleaded fuel to bring the alcohol content lower is recommended. Cold start cranking time should improve. This system is not designed for methanol fuel. Use of methanol fuel will cause the fuel composition sensor to read a higher percentage than that which exists and could cause a DTC P0169 to set, with a DTC P0179 also possible if the concentration is high enough (M85). The wrong fuel (M85) will also cause damage to the fuel system components and should be drained from the tank if found. Other contamination of alcohol blended fuels include water and salts. These compounds, if contained in the fuel, will also cause a higher than expected alcohol content reading causing a DTC P0169 to set. The table below shows the maximum thresholds of a given ethanol content, or percentage, versus the ambient temperature at which a DTC P0169 will set.

The numbers below refer to the step numbers on the diagnostic table.

2. If DTC P0179 is also set, this diagnostic table will not find the cause and may lead to fuel replacement when that may not be appropriate. The technician must follow the diagnostic table for DTC P0179.

3. The frequency read on the scan tool should be stable. A lower reading that momentarily rises above 100 Hz may indicate that water droplets in the fuel are present.