Fuel and Emissions Systems Symptom Troubleshooting

When the vehicle has one of these symptoms, check the diagnostic trouble code (DTC) with the scan tool. If there is no DTC, do the diagnostic procedure for the symptom, in the sequence listed, until you find the cause.

Symptom Diagnostic procedure Also check for 
Engine will not start
(MIL works OK, no DTCs set) 
1.

2.

3. 
Test the battery.
Test the starter.
Troubleshoot the fuel pump circuit. 
  • Low compression
  • No ignition spark
  • Intake air leaks
  • Locked up engine
  • Broken timing chain
  • Contaminated fuel
 
Engine will not start
(MIL comes on and stays on, or never comes on at all, no DTCs set) 
Troubleshoot the MIL circuit.  
Engine will not start
(immobilizer indicator light stays on or flashes) 
Troubleshoot the immobilizer system.  
Hard starting
(MIL works OK, no DTCs set) 
1.

2. 
Test the battery.
Check the fuel pressure. 
  • Low compression
  • Intake air leaks
  • Contaminated fuel
 
Cold fast idle too low
(MIL works OK, no DTCs set) 
1.
2. 
Do the ETCS learn procedure.
Check the idle speed. 
 
Cold fast idle too high
(MIL works OK, no DTCs set) 
1.
2.

3. 
Do the ETCS learn procedure.
Check the idle speed.
Inspect/adjust the throttle cable. 
 
Idle speed fluctuates
(MIL works OK, no DTCs set) 
1.
2.

3. 
Do the ETCS learn procedure.
Check the idle speed.
Inspect/adjust the throttle cable. 
Intake air leaks 
After warming up idle speed is below specifications without load
(MIL works OK, no DTCs set) 
1.
2. 
Do the ETCS learn procedure.
Troubleshoot the alternator FR signal circuit. 
Vacuum hose clogged/cracked/poor connection 
After warming up idle speed is above specifications without load
(MIL works OK, no DTCs set) 
1.
2. 
Do the ETCS learn procedure.
Troubleshoot the alternator FR signal circuit. 
 
Idle speed fluctuates
(MIL works OK, no DTCs set) 
1.
2.

3.


4. 
Do the ETCS learn procedure.
Test the fuel pressure.
Troubleshoot the alternator FR signal circuit.
Inspect and test the PCV valve. 
Contaminated fuel 

Symptom Diagnostic procedure Also check for 
Misfire or rough running
(MIL works OK, no DTCs set) 
1.

2. 
Test the fuel pressure.
Troubleshoot the fuel pump circuit. 
  • Low compression
  • Valve clearance
  • Contaminated fuel
 
Fails emission test
(MIL works OK, no DTCs set) 
1.


2.

3. 
Inspect the three way catalytic converter (TWC).
Test the fuel pressure.
Check the EVAP emission control system. 
  • Contaminated fuel
  • Low compression
  • Broken timing chain
 
Low power
(MIL works OK, no DTCs set) 
1.

2.

3.

4. 
Test the fuel pressure.
Check the air cleaner element.
Inspect/adjust the throttle cable.
Inspect the three way catalytic converter (TWC). 
  • Contaminated fuel
  • Low compression
  • Camshaft timing
  • Engine oil level
 
Engine stalls
(MIL works OK, no DTCs set) 
1.
2.

3. 
Do the ETCS learn procedure.
Test the fuel pressure.
Check the idle speed. 
  • Intake air leaks
  • Faulty harness and sensor connections
 
Difficult to refuel Inspect the breather line and the suction line.