Error Code P091D is defined as ISO/SAE Reserved Mean. Before you try to diagnose a code of this type, it pays to know that the problem is usually less about something being defective or broken than a system not meeting certain standards as set by a pair of regulatory bodies.
The regulatory bodies are the ISO (International Standard of Organization) and the SAE (Society of Automotive Engineers). If the latter abbreviation (SAE) seems familiar, it is likely because these three characters are used to clarify that some tools, nuts, bolts, and washers, are not metric but standard in thread and measure.
Since late 1995, car manufacturers have worked closely with these two regulatory bodies to develop a practical and unwavering set of mandated guidelines that are cataloged and numbered. These guidelines point to the assurance and speed of delivering messages regarding the vehicle’s onboard diagnostic systems and serial data communication. The areas of minimizing conflicting messages, cost efficiency, and electromagnetic field noise suppression are also included in ISO/SAE guidelines.
The CAN (controller area network) bus, the DC bus (a multiplexed communication network), the keyword protocol (allows scanners and other diagnostic devices to communicate with onboard controllers), the local interconnected network (LIN) (responsible for non-critical communication such as entertainment and comfort systems), and the vehicle area network (VAN) are the basic automotive protocols which are subject to the standards set forth by the ISO/SAE.
For this guide, we will focus on the CAN bus; it’s a vast network of connectors and wires that allows the PCM and onboard controllers to communicate with each other instantly and simultaneously. To understand its importance, know that there may be as many as seventy onboard controllers present in a single vehicle.
The PCM (powertrain control module, also known as ECM or engine control module in other vehicles) identifies an interface error in one of the many onboard controllers, or the CAN, which means the ISO/SAE guidelines are not met.
This code activates the Check Engine light and registers the code to the vehicle’s memory system. However, it will not cause any drivability symptoms. Hopefully, other codes will show up along with this code.
Common causes for this code include:
- Damage in the connector or wiring
- Defective controller
- Controller programming error
How to Check
The best thing you can hope for when diagnosing this code and other ISO/SAE codes is another stored code. Codes and symptoms for this type of code usually play a huge part in its diagnosis and repair. Thus, when diagnosing this code, you would have to diagnose the other present code(s) as well.
To diagnose, start by checking the TSB (technical service bulletins) that matches the vehicle’s condition, including symptoms and codes.
Ensure the battery is fully charged, the alternator must be charging adequately, and corrosion in the battery and cable ends are removed. Low voltage condition or poor connection at the battery can result in this code.
Rodent damage can also cause CAN malfunction. Thus, start your visual inspection of all visible connectors and repair as necessary.
After the inspection, connect the scanner to the vehicle’s diagnostic port and retrieve all stored data, as well as the freeze frame data. Write this information down as you may need it as you go on in your diagnosis. Next, clear the codes and then test-drive the vehicle. If the code does NOT reset, then that would mean the problem is intermittent. Use the vehicle normally until the PCM enters readiness mode to reset the ISO/SAE code.
If the code resets and you find no helpful entries from the TSB, perform a wiggle test on all controllers or CAN bus connectors. If you get no results from a wiggle test, there may be a controller defect or controller programming error.
How to Fix
- Remove corrosion in battery’s connectors and cables
- Repair or replace wiring and connectors in the CAN
There are many other ISO/SAE reserved diagnostic OBD-II trouble codes.