Fuel and Emissions Systems Symptom Troubleshooting

When the vehicle has one of these symptoms, check the Diagnostic Trouble Code (DTC) with the HDS or a 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
  • Slipped/ broken timing belt
  • 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.  
Engine is hard to start
(MIL works OK, no DTCs set) 
1.

2. 
Test the battery.
Check the fuel pressure. 
  • Low compression
  • Intake air leaks
  • Contaminated fuel
  • Weak spark
 
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 without load
(MIL works OK, no DTCs set) 
1.

2. 
Troubleshoot the alternator FR signal circuit.
Inspect and test the throttle body. 
Vacuum hose clogged/cracked/poor connection 
After warming up idle speed is above specifications without load
(MIL works OK, no DTCs set) 
Troubleshoot the alternator FR signal circuit.  

*1: except KU, KU (RV-i-H, RV-Si-H models), FO, MA, PH models 
*2: except KU (RV-i-H, RV-Si-H models) model 

Symptom Diagnostic procedure Also check for 
After warming up idle speed drops when steering wheel is turning
(MIL works OK, no DTCs set) 
1.

2. 
Troubleshoot the PSP switch signal circuit.
Inspect and test the throttle body. 
Power steering system 
Idle speed fluctuates
(MIL works OK, no DTCs set) 
1.

2.

3.

4. 
Test the fuel pressure.
Test the injectors.
Troubleshoot the alternator FR signal circuit.
Inspect and test the PCV valve. 
Contaminated fuel 
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
  • Slipped/ broken timing belt
 
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
 

*2: except KU (RV-i-H, RV-Si-H models) model