What is Trouble Code P0491? OBD II Trouble Code P0491 indicates trouble with the Secondary Air Injection System. The components which make up the Air Injection System are the air pump assembly consist of a pump, mass air flow sensor (MAF), relay, bracket, and inlet filter. The air injection system adds a controlled amount of air to your engine's exhaust system aiding the oxidation of hydrocarbons (HC) and carbon monoxide (CO), and is only activated during cold starts.
How does the Air Injection System work? Air is drawn into the air pump through a tube that is connected to the Mass Air Flow Sensor (MAF) and air filter assembly. Air flow is measured by the Mass Air Flow sensor and the voltage produced will be set to the ECU (engine contorl unit). This voltage will be in direct relation to the amount of air flowing through the MAF. The amount of voltage the MAF sends out to the ECU will determine the amount of fuel which will be added to the combustion chambers in order to achieve the air/fuel ratio od 14.7:1. Air is then compressed by the air injector pump and is routed through a tube, through the one-way check valves and injected into the catalytic converters. The one-way check valves protect the hoses, air pump and injection tubes from hot exhaust gases backing up into the air injection system. Air is allowed to flow through these valves in one direction only and only towards the catalytic converter. This downstream air flow assists the oxidation process in the catalyst (CAT).
When is the AIR Injection system tested? Your vehicle's ECU will run a test on the Air Injection System while the engine is running, the air injection pump has been active for a calibrated amount of time, and once enough air flow has accumulated through the Air Injection system.
What will cause a P0491 to set? If the PCM detects excessive air flow or not enough air flow through the Air Injection system, a fault (trouble code) will be recorded. The ECU will test the system twice before storing a P0491 trouble code and turning on the Check Engine Light.
What are some of the possible causes which would trigger an OBDII Trouble Code P0491?
1. Good trip equal to zero
2. ASD relay output circuit
3. MAF sensor internal failure
4. MAF signal circuit shorted to ASD relay output circuit
5. MAF signal circuit shorted to battery voltage
6. MAF signal circuit open
7. MAF signal circuit shorted to ground
8. MAF signal circuit shorted to (K4) sensor ground circuit
9. MAF Sensor ground circuit open
10. Defective PCM
Based on the possible causes above it is obvious although P0491 is Air Injection System related, it is quite possible for a defective Mass Air Flow Sensor (MAF) to be causing the problem. If you've replaced the air pump and Trouble Code P0491 is still popping up we recommend inspecting the MAF for proper operation. Of course remember to check the basics first... Broken disconnected vacuum lines, inlet tube, air injection valve, and so on. Simply to a quick visual make sure everything appears intact.