What is OBD-II Code P2A99 – Alternative Fuel Tank Select Valve C Control Circuit/Open

OBD-II Code P2A99 – Alternative Fuel Tank Select Valve C Control Circuit/Open: Understanding and Repairing the Issue

The On-Board Diagnostics (OBD) system in modern cars is a vital tool for mechanics and drivers alike. This system monitors various components of your car’s engine, exhaust, and other systems to ensure everything is working correctly. When something goes wrong, the OBD system generates a diagnostic trouble code (DTC) to help mechanics pinpoint the exact issue.

One such code is the OBD-II Code P2A99, which indicates a problem with the alternative fuel tank select valve C control circuit/open. If you’re not familiar with car systems, this code may seem like gibberish. In this article, we’ll explain what this code means, its possible causes, and the repair options available.

What is OBD-II Code P2A99?

OBD-II Code P2A99 indicates a problem with the alternative fuel tank select valve C control circuit. Specifically, it means that the ECM (Engine Control Module) has detected an open circuit in this valve’s control circuit. The alternative fuel tank select valve is a component of your car’s fuel system that controls the flow of fuel between the primary and secondary fuel tanks. The valve ensures that your car runs on the right fuel and reduces the risk of accidental fuel mixing.

Possible Causes of OBD-II Code P2A99

Several things can lead to an open circuit in the alternative fuel tank select valve control circuit, resulting in OBD-II Code P2A99. Some of the most common causes are:

1. Faulty valve – A faulty alternative fuel tank select valve could cause an open circuit and trigger this code.

2. Wiring issues – Loose, corroded, or damaged wiring in the valve’s control circuit can prevent a proper circuit connection.

3. ECM software issues – Sometimes, outdated or corrupted ECM software can cause an open circuit. In such a case, it may be necessary to update or replace the ECM.

4. Faulty ECM – An ECM malfunction can inhibit current flow to the valve, causing an open circuit

Diagnosing OBD-II Code P2A99

When the OBD-II system detects a problem, it illuminates the check engine light on your dashboard, indicating there’s an issue. When P2A99 is the DTC in question, a mechanic may perform the following steps to diagnose the problem:

1. Visual inspection – The mechanic can begin by inspecting all components related to the alternative fuel tank select valve C control circuit for visible signs of damage or wear.

2. Use of a scan tool – Next, they will use a scan tool to retrieve the trouble codes and access the live datastream. From there, they can asses the ECM’s commanded operation of the alternative fuel tank select valve C control circuit.

3. Pinpoint Testing – By following manufacturer guidelines, a series of pinpoint tests can be performed to identify specific electrical issues that are causing the valve to malfunction.

4. Resistance test – If the mechanic suspects that there may be an issue with the valve, they may perform a resistance test of the valve and its circuits.

Repair Options

The repair required for OBD-II Code P2A99 will depend on the root cause identified during diagnosis. Here are some of the common repair options available:

1. Repairing or replacing the valve – If the problem is a faulty valve, your mechanic may suggest repairing or replacing the valve.

2. Repairing or replacing the wiring – If the issue is a wiring problem, your mechanic may suggest repairing or replacing the affected wires.

3. Updating or replacing ECM – In some cases, updating or replacing the ECM may be necessary to resolve the issue.

4. Completing manufacturer updates – Manufacturers may have released updates to intoduce better energy efficiency or to improve diagnostic capabilities. Your mechanic may suggest adding these updates to all control modules on your vehicle.

5. Conduct a reset – A reset can be conducted to erase the error code from your car’s computer. In this instance, the engine light will come back if the problem has not been corrected.

Frequently Asked Questions

Q1. What happens if I ignore OBD-II Code P2A99?
Ignoring the code can lead to worsened fuel efficiency, power loss, and engine damage. Additionally, if the code is not resolved, you won’t pass inspections, and you won’t be able to register your vehicle legally.

Q2. Can I continue driving my car with OBD-II Code P2A99?
Yes, there is no risk of immediate danger when you see that this code has been detected. However, repairs should be made as soon as possible, keeping in mind that this code can lead to other issues.

Q3. Can I repair OBD-II Code P2A99 myself?
We cannot recommend that you attempt to repair this code if you don’t have the necessary technical knowledge or expertise. Trying to fix the problem yourself could cause further issues or damage to your vehicle.

Q4. Can OBD-II Code P2A99 be harmless or false?
There is always a possibility that the issue is detected through the OBD-II system, but it may not affect your vehicle’s performance. However, it is always important to address the problem as soon as possible to prevent any potential engine damage from prolonged issues.

Q5. How much will it cost to repair OBD-II Code P2A99?
The cost of repairing OBD-II Code P2A99 may vary depending on the root cause of the problem. The best course of action is to obtain a quotation from a trusted mechanic after diagnosis.


The OBD-II Code P2A99 is one of many codes the OBD system can generate, indicating a problem with the alternative fuel tank select valve C control circuit. With the knowledge and information provided in this article, you should now have a better understanding of what this specific code means, its potential causes and, most importantly, the repair options available. If you encounter this code, it’s best to seek professional help to properly diagnose and repair the issue. Keep in mind that ignoring this code can lead to costly engine damage and inability to legally operate your vehicle.

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