Chart 1 of 3

Diagnostic Chart 1 Part 1:

Diagnostic Chart 1 Part 2:

In order to correctly diagnose a no start condition, battery voltage must be above 11 volts and the engine must be cranking at normal cranking speed (above 200 rpm). Fuel level and fuel quality in the fuel tank should be checked (do not solely rely on the fuel gauge for level reading). When the engine is cranked, the BCM will cheek the password from the passlock sensor. If the password is not received within a certain amount of time or does not match the BCM password, the BCM will send a "fuel disable command" to the ECM over the CAN link. The SECURITY telltale will flash and the engine will not be allowed to start. If the passwords match, the BCM will send the password over the CAN link to the ECM. If the ECM password does not match the BCM password; or if the ECM does not receive a password from the BCM, it will allow the engine to start then stall within four seconds. The SECURITY telltale will flash for 10 minutes if the ECM receives no password; an incorrect password or a fuel disable command. If the password to the ECM is correct, the engine will operate normally. The password is sent continuously by the BCM to the ECM during run mode. If the password becomes lost or incorrect during the drive cycle, the vehicle will be allowed to start on succeeding ignition cycles without the aid of passlock and the SECURITY telltale will be On steady. This mode is called fail enable mode.

DIAGNOSTIC AIDS
BCM or ECM DTCs that may cause a no start and should be diagnosed first:
^ U-type communication: U1000, U1016, U1064, U2105 or U2107
^ BCM/ECM passlock/security system: P1629, P1630, P1631, B1372, B1382, B2947, B2948, B2957, B2958 or B2960
^ ECM internal fault: P0601, P0602, P0603, P0604 or P0606
^ Fuel pump relay coil circuit: P0628 or P0629
^ Crankshaft position sensor circuit: P0335, P0337, or P0338

If no crankshaft position (CKP) sensor signal is detected during engine crank when a valid camshaft sensor signal is received, P0335 will set after a certain period of time. Fuel and spark will NOT be enabled.

If a MAF circuit fault occurs, the ECM will use engine speed and TP angle to calculate engine airflow. The engine will start if this condition occurs.

Make sure ECT sensor and MAF voltage readings on Scan tool with ignition On, engine Off look normal. MAF voltage should be between 0.8 and 1.2 volts with ignition On, engine Off. The ECT and IAT should be within 2°C (4°F) of each other if the vehicle has been sitting overnight. A skewed sensor reading may cause a hard/no start condition.

Check for a flooded engine. NOTE: There is NO clear flood mode on the L81 engine that would cut fuel Off during an engine crank. However, if the accelerator pedal is pressed between 14 and 100% during an engine crank, the throttle blade will open in proportion to the travel of the accelerator pedal. At full accelerator pedal travel during the engine crank, the throttle blade will be open 100%, which may clear a flooded engine.

Make sure the main relay is operating correctly in the UHFB by listening to the relay click when the ignition is turned On. Monitor MAIN RELAY VOLTAGE on Scan tool as the ignition is turned On and Off.

These ECM fuses in the UHFB will cause a no start if open:
^ ECM/TCM 10 amp
^ PWR TRAIN 10 amp
^ FUEL PUMP 10 amp
^ INJ/MAN 10 amp
^ ECM/CAM 15 amp
^ ENG/IGN 15 amp
^ IGNITION 30 amp
^ BATT 40 amp

These fuses in the IPFB will cause a no start if open:
^ BCM(IGN) 10 amp
^ BCM/CLUSTER 10 amp

IMPORTANT:
^ If the camshaft sensor signal is lost, the ECM will provide spark to both mating cylinders as well as default to non-sequential fuel injection. The engine WILL start if this condition occurs.
^ The ECM will only command the fuel pump relay On when the engine is cranking or running.
^ As soon as the ECM receives ignition voltage at the ignition 1 input, it will turn On the main relay driver and ignition voltage will be present on main relay circuits 5290. If the ignition is turned On and the ECM detects no voltage on main relay circuits 5290, it will NOT communicate with the Scan tool and the BCM will set U2105. If main relay voltage is lost AFTER the ignition was turned On, the ECM will continue to communicate with the Scan tool, no U-type DTCs will be set, but the engine will not start.