OBD-II Code P1640: Internal Control Module Fault

What Does P1640 Mean?

P1640 is a manufacturer-specific code with entirely different meanings depending on your car's make. On Volkswagen and Audi, it signals an internal memory (EEPROM) error within the Engine Control Module (ECU). On General Motors vehicles, it points to a fault in a 'Quad Driver Module,' a set of ECU transistors controlling external relays and solenoids. On Hyundai and Kia, it indicates a main power relay failure or immobilizer communication error. On Ford, Jaguar, and Mazda, it is an informational code meaning 'Powertrain DTCs Available in Another Module,' directing you to scan the Body Control Module (BCM) or Dosing Control Unit (DCU) for the root cause.

Technical definition: The official SAE/OBD-II definition for P1640 is manufacturer-controlled. Common definitions include 'Internal Control Module (EEPROM) Error' (Volkswagen/Audi/BMW), 'Quad Driver Module (QDM) Fault' (General Motors), 'Main Relay Malfunction' or 'Immobilizer Error' (Hyundai/Kia), and 'Powertrain DTCs Available in Another Module' (Ford/Jaguar/Mazda).

Can I Drive With P1640?

Yes, But With Caution. You can likely drive the vehicle, but it carries significant risk. The car often enters a reduced-power 'limp mode,' struggles to start, or stalls unpredictably in traffic. On Hyundai or Kia models, an immobilizer-related P1640 prevents the car from starting entirely. Driving with an active fault risks expensive downstream damage to components like the catalytic converter or transmission. Get the vehicle diagnosed immediately to prevent stranding.

Common Causes

• Internal ECU EEPROM Failure (Very Common) — The most frequent cause on VW/Audi models with Bosch EDC16 ECUs. It involves a checksum error, memory chip failure, or cracked solder joints on the circuit board, often triggered by temperature changes or voltage spikes.
• Faulty External Solenoid or Relay (Common) — On GM vehicles, a failing EVAP purge solenoid, torque converter clutch (TCC) solenoid, or boost control solenoid causes a Quad Driver Module fault. The ECU detects an electrical short or open circuit in the component it controls.
• Immobilizer System Desynchronization (Common) — On Hyundai and Kia vehicles, the ECU loses communication with the immobilizer module or fails to recognize the transponder key, resulting in an immediate no-start condition.
• Low or Unstable System Voltage (Common) — A weak battery, failing alternator, or poor ground connection causes voltage fluctuations that corrupt the ECU's memory or disrupt module communication.
• Corrupted ECU Software or Aftermarket Tune (Common) — An incomplete software update or a poorly written aftermarket performance 'chip tune' directly corrupts the ECU memory.
• Wiring Harness or Connector Damage (Less Common) — A short-to-power, open circuit, or corroded pin at the main ECU connector disrupts signals to controlled components.

Symptoms

• Check Engine Light is On — The Malfunction Indicator Lamp (MIL) illuminates on the dashboard.
• Engine Won't Start or Hard Starting — The vehicle cranks but fails to start. This is the primary symptom for Hyundai/Kia (immobilizer fault) and VW/Audi (EEPROM fault).
• Reduced Engine Power ('Limp Mode') — The engine feels sluggish and restricts acceleration as the ECU defaults to a safe mode to protect the powertrain.
• Engine Stalling — The engine stalls unexpectedly, typically when coming to a stop or idling.
• Erratic Automatic Transmission Shifting — On GM vehicles, a P1640 fault related to the TCC solenoid causes shuddering at highway speeds and poor shift quality.
• Flashing Immobilizer/Security Light — The dashboard security light flashes, indicating the vehicle does not recognize the key.

Diagnostic Flowchart

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

Common Fixes & Costs

• Replace Faulty Solenoid (EVAP or TCC) — Parts: $30-$80, Labor: $150-$300, ~1.5 hr book time (Intermediate)
• ECU Reprogramming or Reflashing — Parts: $0, Labor: $150-$400, ~1.5 hr book time (Professional)
• Reprogram Keys or Replace Immobilizer Module — Parts: $0-$400, Labor: $100-$250, ~1.0 hr book time (Professional)
• Repair or Remanufacture the ECU — Parts: $200-$800, Labor: $100-$300, ~1.5 hr book time (Professional)
• Replace the Engine Control Unit (ECU) — Parts: $600-$2000, Labor: $150-$500, ~2.0 hr book time (Professional)
• Repair or Replace Damaged Wiring — Parts: $10-$50, Labor: $150-$500, ~3.0 hr book time (Professional)

Used vs. New Parts: Buying Guide

When a used part is worth it: A used ECU makes sense for older, high-mileage vehicles where a new unit is cost-prohibitive. Buy from a reputable seller offering a warranty and return policy.

Donor-vehicle mileage cap: roughly under 100000 miles for the part to have meaningful remaining life.

Donor quality checklist:

• Match the part number on the ECU case exactly. A single letter difference indicates incompatibility.
• Inspect the connectors for bent, corroded, or missing pins.
• Avoid units with missing stickers or signs of tampering, as they may have a failed aftermarket tune.

Decision logic:

• If The fault is internal to the ECU and a software reflash is not an option. → Choose a remanufactured ECU rebuilt to factory specs with a warranty.
• If The vehicle is over 10 years old and has high mileage. → A used ECU from a trusted source is cost-effective, but factor in programming costs.
• If The vehicle is newer or has low mileage. → A new OEM ECU ensures longevity and avoids compatibility headaches.

Warranty tradeoff: Used ECUs from salvage yards are sold 'as-is' or with a 30-day warranty. Remanufactured units offer a 1-year warranty. A new OEM ECU includes a parts and labor warranty.

Worst-case if a used part fails: $400-$1000 if a used ECU is faulty, covering repeat labor and failed programming services.

What Happens If You Wait — Timeline

1. 0-1 month: Check Engine Light appears. VW/Audi experiences intermittent no-starts. GM shows no symptoms. Hyundai/Kia suffers an immediate no-start. (MPG impact: 0-5%% · Added cost: $0)
2. 1-3 months: Symptoms solidify. VW/Audi no-starts become frequent. GM TCC faults cause highway shuddering and poor fuel economy. (MPG impact: 5-10%% · Added cost: $100-$300)
3. 3-6 months: Downstream damage begins. GM transmissions overheat from slipping torque converters, degrading fluid and entering limp mode. (MPG impact: 10-15%% · Added cost: $600-$1,500)
4. 6+ months: Catastrophic failure. GM vehicles require a full transmission rebuild due to destroyed internal clutch packs. VW/Audi vehicles become permanently inoperable. (MPG impact: 15-25%% · Added cost: $2,500-$5,000+)

Cost of Not Fixing It

• Immediate: Risk of the vehicle not starting or stalling in traffic, creating a major safety hazard. The vehicle enters a reduced-power 'limp mode.' (Added cost: Potential towing costs ($100-$300).)
• 1-3 months: Continued driving with a faulty component places stress on other systems. A GM car with a faulty TCC solenoid causes excessive wear on the transmission torque converter. (Added cost: $500-$2000 (for transmission or torque converter repairs).)
• 3+ months: If the fault causes an overly rich or lean fuel mixture, it destroys the catalytic converter. (Added cost: $1200-$2800 (for catalytic converter replacement).)

Diagnosis Steps

1. Perform a Full-System Diagnostic Scan
   Use a capable OBD-II scanner to read codes from ALL modules. On Ford, Jaguar, and Mazda, P1640 is an informational code indicating the real fault is stored elsewhere (e.g., BCM, TCM). You must scan every module to find the source.
   Tools: Professional OBD-II Scanner (Beginner)
2. Inspect Battery and Charging System
   Ensure the battery is fully charged (12.4-12.6V engine off) and the charging system works correctly (13.5-14.5V engine running). Low voltage is a primary cause of ECU memory errors. Perform a load test on the battery.
   Tools: Digital Multimeter, Battery Load Tester (Beginner)
3. Test Related External Components (GM, Hyundai/Kia)
   For GM vehicles, test the resistance of components controlled by the Quad Driver Module (EVAP purge, TCC solenoid). A typical solenoid reads 20-40 ohms. For Hyundai/Kia, test the main relay (80-120 ohms across coil pins) and check for a valid immobilizer signal.
   Tools: Digital Multimeter, Vehicle-Specific Scan Tool (Intermediate)
4. Verify ECU Power and Ground Signals
   Use a wiring diagram to check power and ground signals directly at the ECU connector. Check for battery voltage on power input pins. Check for voltage drop on ground pins; a reading over 100mV indicates a poor ground connection requiring repair.
   Tools: Digital Multimeter, Vehicle-Specific Wiring Diagram, Back-Probe Pins (Advanced)
5. Check for Available ECU Software Updates
   For VW/Audi and other makes, a known software bug causes P1640. A dealership or specialized shop checks if a factory software update (reflash) is available for your ECU.
   Tools: Manufacturer-Specific Scan Tool (e.g., VCDS, ODIS) (Advanced)
6. Visually Inspect Wiring and Connectors
   Inspect the wiring harness leading to the ECU and related components. Look for damage, corrosion, loose connections, or chafing. Pay close attention to the main ECU connector and chassis ground points.
   Tools: Flashlight, Inspection Mirror (Intermediate)
7. [PRO TIP] ECU Environmental Stress Test (VW/Audi)
   For intermittent non-start issues on older VW/Audi models, use electronics freezer spray on the ECU's main EEPROM chip. If the car starts, it confirms a bad solder joint that makes contact when cold. Warming the ECU with a heat gun can also trigger or resolve the fault, confirming internal hardware failure.
   Tools: Electronics Freezer Spray, Heat Gun (Professional)
8. [PRO TIP] Quad Driver Module Circuit Analysis (GM)
   For a GM QDM fault, use an oscilloscope to monitor the control circuit of the suspected component. When the ECU commands the component on, voltage should drop sharply to near zero. If voltage stays high, it indicates an open circuit. If it stays at zero, it suggests a short to ground.
   Tools: Automotive Oscilloscope, Ammeter (Professional)

When This Code Triggers (Freeze-Frame Conditions)

• Engine State: Key On, Engine Running or during startup (The EEPROM checksum test runs at startup, triggering the code before driving. On GM, it triggers when a specific solenoid activates.)
• Engine RPM: 700-2500 RPM (Occurs at idle, during startup self-tests, or under steady-state cruise when a component controlled by a Quad Driver is commanded on.)
• Engine Load: 15-50% (Occurs under light to moderate load during normal driving, not typically associated with high-performance conditions.)
• System Voltage: 11.5-14.5V (Triggered by a voltage spike during a jump start or low voltage from a failing battery, corrupting internal memory.)

Related Codes

• P1603 — On VW/Audi, P1603 ('Internal Control Module Malfunction') often appears with P1640. Seeing them together confirms an ECU hardware or software failure.
• U1000 / U1255 — On Ford or GM, P1640 acts as a signpost. If you find a U-code in another module, that is the root problem. Diagnose the vehicle's data bus wiring, not the engine.
• P0740 — On GM vehicles, P1640 indicates a Quad Driver Module fault. A companion P0740 pinpoints the exact failure to the Torque Converter Clutch (TCC) solenoid circuit.
• P22FB — On Mazda diesels, P1640 in the PCM is accompanied by P22FB in the Dosing Control Unit (DCU). P1640 simply tells you the DCU stored an emissions code.

Climate & Environmental Factors

• Cold Temperatures: Cold causes hairline cracks in ECU solder joints to contract and create an open circuit. This is a well-documented cause of intermittent no-start conditions on older VW/Audi models.
• High Humidity and Moisture: Prolonged exposure to moisture accelerates corrosion on ECU connectors and chassis ground points, increasing electrical resistance and triggering communication errors.
• Extreme Heat: Sustained high temperatures degrade electronic components within the ECU, leading to premature failure of capacitors and memory chips.

How to Talk to a Mechanic About This Code

Say this: "I have a P1640 code on my [Make/Model/Year]. I know this code has different meanings, so I'd like a diagnosis that starts with the most likely cause for my specific vehicle. If it's a GM, please check the external solenoids first. If it's a Ford, please perform a full-system scan to find the code in the other module."

This signals you understand the code's complexity. It directs the technician toward a logical diagnostic path rather than immediately defaulting to an expensive ECU replacement.

Avoid saying:

• 'Just fix my check engine light.'
• 'My car has an internal computer fault, can you replace it?'
• 'I think I need a new computer.'

Questions to ask before authorizing the repair:

• Based on my car's make, what specific diagnostic path did you follow to arrive at this conclusion?
• If recommending an ECU replacement, did you confirm all power/ground signals and check for software updates?
• If the repair involves programming, is that included in the quote and do you have the specific tools to do it?
• What is the warranty on the recommended parts and labor?

Where to Take It: Dealer vs Independent vs Chain

• Dealer: Recommended for software, immobilizer, or internal ECU faults on German makes. Necessary if ECU replacement and programming are required.
  Best for: Vehicles under warranty., Complex ECU programming, software updates, and immobilizer issues on European makes (VW, Audi, BMW)., When a manufacturer-specific Technical Service Bulletin (TSB) exists.
  Downsides: Highest labor rates., Defaults to replacing the ECU rather than attempting a nuanced repair like re-soldering. (Typical cost: +50% vs. baseline)
• Independent Shop: Best fit for GM and Asian vehicles where the fault is external to the ECU. A specialist independent shop is a better value than a dealer for out-of-warranty repairs.
  Best for: Out-of-warranty vehicles., Diagnosing and replacing external components (e.g., a TCC solenoid on a GM)., Shops specializing in your vehicle's make.
  Downsides: May lack expensive, manufacturer-specific programming tools., Quality varies widely; vet shops based on reviews and ASE certifications. (Typical cost: +0% vs. baseline)
• Chain Shop: AVOID for P1640 diagnosis and repair. They are not equipped to handle the complexities of this code.
  Best for: Checking battery and charging system health.
  Downsides: Technicians lack specialized training for complex module-level diagnostics., Highly likely to misdiagnose P1640 as needing an ECU without proper investigation. (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the total estimated repair cost exceeds 40-50% of the car's private-party value, seriously consider selling or trading in the vehicle.

• Car worth $3000, fix is $1800: Walk away. The repair cost is 60% of the car's value, making it a poor investment.
• Car worth $8000, fix is $1200: Fix it. The repair is well below the 40-50% threshold and extends the car's life significantly.
• Car worth $4500, fix is $2200: Borderline. At nearly 50% of the car's value, get a second opinion and carefully consider the car's overall condition before proceeding.

What Scan Tool You Need for This Code

Minimum: A scanner that reads manufacturer-specific codes from ALL vehicle modules (Engine, Body, Transmission).

A basic $20 code reader only checks the engine module. For Ford and Mazda, P1640 is an informational code, and the real fault is in another module a basic reader can't see. You will waste money guessing.

Budget: BlueDriver Pro Scan Tool (~$119) — Reads codes from all modules and provides freeze-frame data. This is enough to see the full picture on a Ford/Mazda or spot related codes on a GM.

Mid-range: Foxwell NT510 Elite (~$150) — Provides full-system diagnostics and bidirectional controls. For a GM P1640, this allows you to command specific solenoids to see if they work, confirming the fault without replacing parts.

Professional: Autel MaxiCOM MK808S (~$450) — Offers extensive bidirectional controls, all-module scanning, and advanced functions like key programming. Necessary for diagnosing immobilizer faults on Hyundai/Kia.

Rent vs buy: For a one-time diagnosis, auto parts stores offer free scanning services. However, their tools may not read all modules. Investing in a mid-range tool is more effective than paying for an hour of shop diagnostic time.

How to Clear the Code After You Fix It

1. Use a compatible OBD-II scan tool to clear all fault codes.
2. Perform a complete drive cycle to allow the ECU to run its self-tests and set the readiness monitors.

Drive cycle (~30 minutes): Start with a cold engine. Idle for 3 minutes. Drive for 10 minutes in stop-and-go city traffic. Drive for 15 minutes at a steady highway speed (55-60 mph). Allow the vehicle to cool down completely. Fuel tank must be between 1/4 and 3/4 full for the EVAP monitor to run.

Readiness monitors affected: Comprehensive Component Monitor, Misfire Monitor, Catalyst Monitor, EVAP System Monitor

Before emissions retest: drive at least 100 miles to fully set monitors.

Watch out for:

• Clearing the code does not fix the underlying issue; the code returns immediately if the root cause remains.
• Disconnecting the battery resets all readiness monitors to 'Not Ready', causing an automatic emissions test failure.

Will This Fail Emissions / State Inspection?

Yes — this code typically fails an OBD-II emissions inspection.

• California: An illuminated Check Engine Light is an automatic failure for a SMOG check. A full drive cycle must be completed to set readiness monitors before a re-test.
• New York: The NYS inspection includes an OBD-II scan. A P1640 code results in an emissions failure.
• Texas: In the 17 counties requiring emissions testing, an active P1640 code causes the vehicle to fail the OBD inspection.

Most Commonly Affected Vehicles

• Volkswagen Golf, Jetta, Touran, Caddy, Transporter (2003-2009) — Extremely common on 1.9L and 2.0L TDI models with the Bosch EDC16U1 ECU, which is prone to EEPROM failure.
• Audi A3 (2003-2012) — Affects TDI models using the Bosch EDC16U1 ECU. Presents as an intermittent no-start sensitive to temperature.
• General Motors (Buick, Pontiac, Chevrolet) Various models with 3.4L and Supercharged 3.8L engines (1994-2005) — Appears as a Quad Driver Module (QDM) fault related to external components like the EVAP purge or TCC solenoid.
• Hyundai / Kia Terracan, Carnival, Santa Fe, Sorento (2002-2010) — Indicates a fault with the main relay or the immobilizer system failing to recognize the key. Often misdiagnosed as a bad ECU.
• BMW 3-Series (E46), 5-Series (E39), X5 (E53) (1998-2006) — Signifies an internal ECU (DME) memory fault, often triggered by a bad aftermarket tune or low battery voltage.
• Honda / Acura Civic, Accord, MDX, TL (2001-2007) — Points directly to an internal hardware failure within the Engine Control Module (ECM/PCM).
• Volkswagen Jetta, Passat, Beetle, Golf (2011-2014) — TSB 01-17-05TT notes P1640 sets on 2.5L engines after ECU replacement due to incorrect coding.
• Mazda CX-5 (2018-2019) — On SKYACTIV-D 2.2L diesels, P1640 is an informational code pointing to DTCs in the Dosing Control Unit (DCU). TSB R004/22 addresses this with a software update.
• Jaguar Various Models (2000-2010) — Means 'Powertrain DTCs Available in Another Module.' Requires scanning the Body Control Module (BCM) to find the actual fault.

Manufacturer-Specific Notes

• Volkswagen/Audi: The code almost always points to an internal EEPROM error in the Bosch EDC16 engine computer, especially on TDI diesels.
• General Motors: The code means 'Quad Driver Module (QDM) Fault.' The fault is almost always in an external solenoid, and the ECU itself is perfectly fine.
• Ford/Jaguar/Mazda: P1640 is an informational code meaning 'Powertrain DTCs Available in Another Module.' It tells the technician to scan other computers to find the root cause.
• Hyundai/Kia: The definition is 'Main Relay Malfunction' or an immobilizer communication error. This causes a no-start condition frequently misdiagnosed as a bad ECU.

Real Owner Stories

2005 VW Jetta TDI with 155K miles - Intermittent No-Start

The owner experienced an intermittent no-start condition on cold mornings. The car cranked but wouldn't fire. After warming up in the sun, it started perfectly. The only code was P1640.

What they tried:

1. Tested the battery and alternator (both passed).
2. Replaced the main ECU power relay.
3. Received a $1,500 quote for a new ECU and programming.

Outcome: The owner removed the ECU and gently warmed it with a hairdryer, causing the car to start immediately. Using electronics freezer spray on the main EEPROM chip replicated the no-start instantly, confirming cracked solder joints. They sent the ECU to a repair service for $250 to have the chip re-soldered. The car has started reliably for two years.

Lesson: For intermittent, temperature-sensitive P1640 faults on older VW/Audi models, the cause is usually bad solder joints inside the ECU. An environmental stress test with heat or cold confirms this and saves you from buying a new ECU.

2004 Pontiac Grand Prix GTP at 120K miles - Misdiagnosis

The Check Engine Light illuminated with codes P1640 (Quad Driver Module Fault) and P0740 (TCC Solenoid Circuit). The car shuddered at highway speeds.

What they tried:

1. A general mechanic saw the P1640 'Internal Module Fault' and condemned the ECU.
2. The owner received a $900 quote for a remanufactured ECU.

Outcome: Skeptical, the owner researched the codes. On GM vehicles, P1640 indicates a fault in an external component controlled by the ECU. The P0740 code pinpointed the Torque Converter Clutch (TCC) solenoid. A transmission shop replaced the TCC solenoid for $350. Both codes cleared permanently.

Lesson: On GM vehicles, never replace the ECU for P1640 without diagnosing the external components it controls. Companion codes like P0740 tell you exactly which circuit failed.

2014 Hyundai Sonata at 80K miles - No-Start After Battery Died

After a mobile service replaced a dead battery, the car refused to crank. The dashboard showed a solid immobilizer warning light, and a scanner pulled code P1640 ('Immobilizer Error').

What they tried:

1. Tried the spare key fob.
2. Disconnected the new battery for 30 minutes to reset the system.
3. The mechanic suggested towing it to the dealer for a new ECU.

Outcome: The owner hired an automotive locksmith. The locksmith's scan tool revealed the security module lost synchronization with the keys during the battery failure. A 15-minute 'key relearn' procedure fixed the issue for $150.

Lesson: On Hyundai/Kia vehicles, a P1640 no-start after a battery replacement is almost always an immobilizer sync issue. Reprogram the keys before suspecting a damaged ECU.

How to Prevent This Code From Triggering

• Maintain a Healthy Battery and Charging System (Test annually) — Unstable system voltage is the primary cause of ECU EEPROM corruption. A weak battery causes voltage drops during startup that corrupt memory.
• Use Proper Jump-Starting Technique (Every time a jump-start is needed) — Improper jump-starting sends voltage spikes that instantly destroy the ECU. Always connect the final negative cable to a solid chassis ground to prevent sparks.
• Keep ECU and Connectors Dry (Check during other maintenance) — Water intrusion from clogged cowl drains corrodes ECU connector pins. This creates high resistance and short circuits that mimic an internal ECU failure.
• Be Cautious with Aftermarket ECU Tuning (During any software modification) — A 'chip tune' directly modifies the ECU's memory. A bad file or interrupted flash process directly causes P1640. Always use a reputable tuner and a stable power supply.

Frequently Asked Questions

Can a bad battery cause a P1640 code?

Yes. A weak battery or improper jump-starting causes voltage spikes that corrupt the ECU's memory. This disrupts module communication and triggers a P1640 code. Always test your battery and alternator first.

What are the most common misdiagnoses for P1640?

The most frequent mistake is immediately replacing the ECU. On GM vehicles, the cause is usually a cheaper external solenoid, while on Fords, the code simply points to a fault in another module. Always diagnose the specific manufacturer definition before buying parts.

The P1640 code appeared right after my car was 'chip tuned.' Is that related?

Yes. A poorly written or improperly flashed aftermarket performance tune directly corrupts the ECU's EEPROM memory. This is a primary cause of P1640 on Volkswagen, Audi, and BMW vehicles.

What does 'Powertrain DTCs Available in Another Module' mean?

This definition means the Engine Control Module detected a fault request from a different computer on the vehicle's network. P1640 acts as a signpost directing you to the real problem. You must use a full-system scanner to read the other modules and find the root code.

Is it safe to clear the P1640 code?

You can clear the code to see if it was a temporary voltage glitch. However, if it returns immediately, you have a hard fault that requires diagnosis. Ignoring a persistent P1640 leads to stalling or a complete no-start condition.

What is an EEPROM error?

EEPROM is the memory chip inside your car's computer that stores vital data like the VIN, immobilizer keys, and software coding. An error means this data is corrupted, unreadable, or physically damaged. This requires a software reflash or physical repair of the ECU circuit board.

Do I have to go to the dealer to fix a P1640 code?

Not always, but highly likely for German vehicles requiring ECU software updates or immobilizer programming. Independent shops specializing in your vehicle make often have the necessary dealer-level tools for less money. General mechanics usually cannot perform these specific module programming tasks.

Key Takeaways

• P1640 definitions vary by manufacturer; on Ford and Mazda, it is an informational code requiring a full-system scan to find the actual fault in another module.
• On 2003-2012 VW and Audi models, P1640 almost always indicates an internal EEPROM failure requiring a $200-$800 ECU repair or replacement.
• On GM vehicles, P1640 points to a Quad Driver Module fault, meaning an external component like a $30-$80 TCC or EVAP solenoid has failed, not the ECU itself.
• On Hyundai and Kia vehicles experiencing a no-start with P1640, test the 80-120 ohm main power relay and immobilizer sync before condemning the engine computer.