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) Check the idle speed.
Cold fast idle too high
(MIL works OK, no DTCs set) 1.
2.
3. Check the idle speed.
Inspect/adjust the throttle cable.
Inspect and test the throttle body.
Idle speed fluctuates
(MIL works OK, no DTCs set) 1.
2.
3. Check the idle speed.
Inspect/adjust the throttle cable.
Inspect and test the throttle body. Intake air leaks
After warming up idle speed is below specifications with no load
(MIL works OK, no DTCs set) 1.
2. Troubleshoot the ALT FR signal circuit.
Inspect and test the throttle body. Vacuum hose clogged/cracked/poor connection
After warming up idle speed is above specifications with no load
(MIL works OK, no DTCs set) Troubleshoot the ALT FR signal circuit.
Idle speed fluctuates
(MIL works OK, no DTCs set) 1.
2.
3.
4. Test the fuel pressure.
Test the injectors.*
Troubleshoot the ALT FR signal circuit.
Inspect and test the PCV valve. Contaminated fuel

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)	Check the idle speed.
Cold fast idle too high (MIL works OK, no DTCs set)	1. 2. 3.	Check the idle speed. Inspect/adjust the throttle cable. Inspect and test the throttle body.
Idle speed fluctuates (MIL works OK, no DTCs set)	1. 2. 3.	Check the idle speed. Inspect/adjust the throttle cable. Inspect and test the throttle body.	Intake air leaks
After warming up idle speed is below specifications with no load (MIL works OK, no DTCs set)	1. 2.	Troubleshoot the ALT FR signal circuit. Inspect and test the throttle body.	Vacuum hose clogged/cracked/poor connection
After warming up idle speed is above specifications with no load (MIL works OK, no DTCs set)	Troubleshoot the ALT FR signal circuit.
Idle speed fluctuates (MIL works OK, no DTCs set)	1. 2. 3. 4.	Test the fuel pressure. Test the injectors.* Troubleshoot the ALT 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.
3.
4. Check the spark plugs.*
Test the fuel pressure.
Test the injectors.*
Troubleshoot the fuel pump circuit.
Low compression
Valve clearance
Contaminated fuel

Fails emission test
(MIL works OK, no DTCs set) 1.

2.
3.
4.
5. Inspect the three way catalytic converter (TWC).
Check the spark plugs.*
Test the fuel pressure.
Test the injectors.*
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.
5.

6. Test the fuel pressure.
Check the air cleaner element.
Inspect/adjust the throttle cable.
Inspect and the test the throttle body.
Inspect the three way catalytic converter (TWC).
Test the injectors.*
Contaminated fuel
Low compression
Camshaft timing
Engine oil level

Engine stalls
(MIL works OK, no DTCs set) 1.
2.
3.

4. Test the fuel pressure.
Check the idle speed.
Troubleshoot the brake pedal position switch signal circuit.
Check the spark plugs.*
Intake air leaks
Faulty harness and sensor connections

Difficult to refuel Inspect the breather line and the suction line.

Symptom	Diagnostic procedure	Also check for
Misfire or rough running (MIL works OK, no DTCs set)	1. 2. 3. 4.	Check the spark plugs.* Test the fuel pressure. Test the injectors.* Troubleshoot the fuel pump circuit.	Low compression Valve clearance Contaminated fuel
Fails emission test (MIL works OK, no DTCs set)	1. 2. 3. 4. 5.	Inspect the three way catalytic converter (TWC). Check the spark plugs.* Test the fuel pressure. Test the injectors.* 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. 5. 6.	Test the fuel pressure. Check the air cleaner element. Inspect/adjust the throttle cable. Inspect and the test the throttle body. Inspect the three way catalytic converter (TWC). Test the injectors.*	Contaminated fuel Low compression Camshaft timing Engine oil level
Engine stalls (MIL works OK, no DTCs set)	1. 2. 3. 4.	Test the fuel pressure. Check the idle speed. Troubleshoot the brake pedal position switch signal circuit. Check the spark plugs.*	Intake air leaks Faulty harness and sensor connections
Difficult to refuel	Inspect the breather line and the suction line.

*: KU model