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 check the password from the passlock sensor. If the password is not received within a certain amount of time or does the 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: P1630, P1631, P1632, B1372, B1382, B2947, B2948, B2957, B2958 or B2960
^ ECM internal fault: P0601, P0602, P0603, P0604 or P0606
^ Fuel pump relay coil circuit: P0230, P0231 or P0232
^ Crankshaft position sensor circuit: P0335
^ Camshaft position sensor circuit: P0341, P0342 or P0343

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

If a MAF circuit fault occurs, the ECM will use the MAP, 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.

Make sure the EGR valve is not stuck open by reading the EGR SENSOR voltage on Scan tool. Normal closed valve readings should be within 0.5-1.0 volts.

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. If the fuel pump operates normally, the main relay circuit is OK.

These ECM fuses in the UHFB will cause a no start if open:
^ BCM/ECM/CRUISE/ABS 10 amp
^ CONTROLS B+ 10 amp
^ ENGINE CNTL 2 10 amp
^ ENGINE CNTL 3 10 amp
^ BCM 15 amp
^ FUEL PUMP 15 amp
^ IGN 15 amp
^ IGN 1/2 30 amp

IMPORTANT: The ECM will only command the fuel pump relay On when the engine is cranking or running.