P1749 on 2013-2014 Audi A8 4.0T: TCM Incorrect Coding Causes and Fixes

What's Unique About the 2013-2014 Audi A8

For the D4 generation Audi A8 and its platform mates (S6, S7, S8), this code is the subject of a specific Technical Service Bulletin (TSB). Audi documented that during a routine software update, the TCM's self-coding function can fail, logging P174900. Unusually, the TSB states that this specific failure often causes no check engine light or noticeable driving problems, making it a 'ghost' code that only appears during a diagnostic scan. However, some owners have reported an amber EPC light or even a Check Engine Light (MIL) with this code, indicating real-world symptoms can vary.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• No Check Engine Light and no noticeable performance issues (official TSB symptom).
• Amber EPC (Electronic Power Control) light on the instrument cluster.
• Slight jolt when shifting from Park into Drive.
• Acceleration may feel less smooth than normal.
• MIL (Check Engine Light) may be illuminated in some cases, contrary to the TSB.

Most Likely Causes

1. Incomplete or Failed TCM Software Update 🔴 High Probability This is a known glitch documented by Audi in TSB #37 14 36 2037151, where the TCM's self-coding function may not complete during a Software Version Management (SVM) update. TSB #2037137 also confirms this can happen as a side effect of other prescribed updates.
   How to confirm: The code appears immediately after a dealer or shop has attempted an SVM update on the vehicle. A scan will show DTC P174900 in the Transmission Control Module (address 02).
   Typical fix: A technician must manually re-code the TCM using the Audi ODIS diagnostic tool, following the specific steps outlined in TSB 2037151. This involves changing Byte 2 to a hex value of '01'.
   Est. part cost: $0
2. Low Battery Voltage or Power Interruption ⚪ Low Probability Sensitive electronic modules like the TCM can sometimes lose or corrupt their coding if battery voltage drops significantly during operation or a software procedure. All Audi software updates require a stable power supply connected to the vehicle.
   How to confirm: Test the vehicle's battery and charging system. If the battery is weak or failing, this could be the root cause, especially if the fault appeared without a recent software update. A VCDS scan log for this fault showed a terminal 30 voltage of 12.382V, which is on the lower side and could indicate a weak battery contributing to the issue.
   Typical fix: Charge or replace the battery, then have the TCM re-coded per the TSB procedure.
   Est. part cost: $200-$500

Rare But Worth Checking

• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly While rare, it's possible the TCM itself has an internal fault that prevents it from accepting or retaining the correct coding. This is usually diagnosed only after multiple attempts to re-code it have failed. The TCM is integrated into the mechatronic unit inside the transmission.

Diagnosis Steps

1. Connect a professional-grade scan tool (like VCDS or ODIS) to the vehicle's OBD-II port.
2. Scan the Transmission Control Module (address word 02) for fault codes.
3. Confirm that DTC P174900 (Automatic transmission control module incorrect code) is stored.
4. Check the vehicle's service history to see if a software update was recently performed.
5. If so, follow the service procedure in Audi TSB #2037151 to manually re-code the TCM using ODIS. The specific steps are:
6. a. In ODIS, select 'Transmission Control Module'.
7. b. Right-click and select 'Control module OBD'.
8. c. In the OBD functions menu, select 'Code' then 'Perform'.
9. d. In the binary coding screen, select the row for 'Byte no. 2'.
10. e. In the pop-up menu, enter '01' in the Hex Entry field and select 'Apply'.
11. f. Select 'Apply' on the main coding screen to write the new value to the module.
12. Clear the P1749 fault from the memory.
13. Cycle the ignition and perform an SVM specified-actual comparison to verify the fix and ensure the code does not return.
14. If manual coding fails repeatedly, investigate the TCM for a potential hardware fault.

Parts You'll Likely Need

• Transmission Control Module (TCM) / Mechatronic Unit (OEM #0BK 927 156 AG) — This part is only needed in the rare event that the original module is faulty and cannot be re-coded. The primary fix is a software procedure, not a part replacement. The part number is for the complete Mechatronic assembly which houses the TCM.
  Trusted brands: Genuine Audi
  OEM price range: $1500-$2500
  Aftermarket price range: $800-$1500

Technical Service Bulletins (TSBs) & Recalls

• 37 14 36 2037151: 'Incorrect coding in control module after software update (DTC P1749 stored in TCM)'. This is the primary document defining the problem and the exact, manual ODIS procedure to fix it.
🎬 Watch: A demonstration of programming an Audi transmission using ODIS
• 37 14 37 2037137: 'Hesitation or delay during the 1-2 upshift'. This TSB mentions that P1749 can be a side effect of its software update procedure and refers technicians to the first TSB for the solution.

Platform-Specific Known Issues

• Audi TSB #37 14 36 2037151 specifically addresses P1749 on the 2013-2014 A8 4.0T. It identifies the cause as a failure of the TCM's self-coding function during an update and provides a manual coding procedure (changing Byte 2 to '01') as the official fix.
• Another TSB (#2037137) for a hesitation issue notes that P1749 may appear as a side effect of its prescribed software update (to versions like 4H1927158EF), reinforcing the link between this code and service procedures. It explicitly instructs technicians to perform TSB 2037151/1 if P1749 appears.

Mechanic-Grade Diagnostic Values

• TCM (J217) CAN Bus Terminal Resistance — expected: 60 to 72 ohms (at approx. 20° C). Failure: A reading outside this range indicates a problem with the CAN bus wiring or the terminating resistor in the ECM.
• Battery Voltage During Coding — expected: At least 12.5 volts. Failure: Voltage below 12.5V can cause data corruption or failed procedures. All official Audi updates require a stable power supply.

Scan Tool Commands That Help

• ODIS (Official Audi Software): Control module OBD -> Code -> Binary coding — This is the specific, manual function required by TSB #2037151 to fix the P1749 fault by changing Byte 2 to '01'. It is used when Guided Functions cannot correct the coding.
• VCDS (VAG-COM): Long Coding — Advanced DIYers may be able to use the Long Coding function in VCDS to directly change the coding bytes of the transmission module (Address 02). This is not the official Audi procedure but may achieve the same result. For some modules on newer Audis, the hood must be open to allow coding changes.

Wiring & Ground Locations

• TCM (J217) Location — The TCM is integrated into the mechatronic unit, which is bolted to the underside of the transmission housing inside the oil pan.. While P1749 is a software code, a major physical issue with the module or its connector (e.g., fluid intrusion) could theoretically cause coding corruption, though this is highly unlikely to be the primary cause.
• Main Harness Ground Points — Wiring diagrams for similar Audi platforms show primary earth connections for the main harness on the bulkhead and center tunnel.. A poor ground connection for the TCM or related modules could potentially cause communication errors or data corruption during a software update, leading to this fault. Checking main ground straps for corrosion is a valid, though low-probability, diagnostic step if software fixes fail.

Real Owner Repair Stories

• AudiWorld Forums user MP4.2+6.0 (2014 Audi A8 4.0T) — Check Engine Light (MIL) was on. The VCDS scan showed fault P1749 00 - Transmission Control Unit Incorrectly Coded - MIL ON.
  ❌ Tried (didn't work) The user first scanned the car and found the code. The immediate advice was to clear it and see if it returned.
  ✅ What actually fixed it The user did not post a final resolution, but the thread confirms a real-world case where P1749 did illuminate the MIL, contrary to the TSB, and provided the exact VCDS fault log. The recommended fix from other users was to take it to the dealer for re-coding, aligning with the TSB.