The B1681 Ford Code indicates a problem with the Passive Anti-Theft System (PATS). This often manifests as a no-crank condition, accompanied by a rapidly flashing security light. This article details a diagnostic process and solution for a Ford vehicle exhibiting these symptoms, specifically focusing on the B1681 fault code: “Passive Anti Theft System (PATS) Transceiver Mod Signal Not Received.”
Understanding the B1681 Ford Code and PATS System
The vehicle in question displayed a no-crank condition and a rapidly flashing security light. Diagnostic trouble codes (DTCs) retrieved from the Powertrain Control Module (PCM) included B1681 and B2103 (“Passive Anti Theft System (PATS) Transceiver Circuit Fault”). Clearing the codes resulted in the B1681 code immediately returning. Attempts to start the vehicle with both available keys were unsuccessful.
The PATS system prevents unauthorized vehicle operation by verifying a valid key. Communication between the PATS transceiver module (located near the ignition key cylinder) and the PCM is crucial for this process. Power and ground circuits at the PATS transceiver were tested and found to be within specifications.
Diagnosing the PATS Communication Failure
Further diagnostics involved examining the data communication lines between the PATS transceiver and the PCM using an oscilloscope. The PCM was observed to be sending requests to the PATS module, but the module failed to respond.
The communication process involves two wires:
- Gray/Orange: 12V output from the PCM to the PATS module. The PATS module should toggle this line to ground to send the key code information back to the PCM.
- White/Light Green: 12V output from the PATS transceiver to the PCM. The PCM toggles this line to ground to request the key code.
The oscilloscope capture revealed the PCM (green trace) requesting the key code, but the PATS transceiver (yellow trace) remained at 12 volts, indicating no response. This lack of communication confirmed a faulty PATS transceiver. A detailed description of the PATS communication protocol can be found here: Description of PATS communication circuits between PATS transceiver and PCM
Replacing the PATS Transceiver
Based on the diagnostic findings, the PATS transceiver was replaced. After clearing the stored codes, the vehicle started successfully with both keys.
This vehicle’s PCM stores the PATS key codes, with the transceiver acting as a sensor to relay the information. Therefore, no key relearning was required after replacing the transceiver. Furthermore, the PATS transceiver on this model is not programmable. The solution involved a simple replacement of the faulty component and clearing the related DTCs.
Conclusion: Resolving the B1681 Ford Code
The B1681 Ford code signifies a communication breakdown between the PATS transceiver and the PCM. Diagnosing this issue involves verifying power and ground at the transceiver and using an oscilloscope to analyze the communication signals. In this case, a faulty PATS transceiver was identified and replaced, successfully resolving the no-crank condition and eliminating the B1681 code. This highlights the importance of systematic diagnostics in addressing PATS-related issues in Ford vehicles.
