OBD-II Code P1668: Comprehensive Diagnostic Guide

What Does P1668 Mean?

P1668 is a manufacturer-specific code signaling a communication breakdown between the Powertrain Control Module (PCM) and the module firing the fuel injectors (IDM or FICM). When these computers lose connection, the engine stops delivering fuel. On Jaguar and Land Rover, this code flags an anti-theft system lockout. On Volkswagen or Audi, it indicates an electrical short in the cylinder 3 fuel injector circuit.

Technical definition: The SAE definition for P1668 varies by manufacturer. The primary definition is 'Powertrain Control Module (PCM) to Injector Driver Module (IDM) Communication Error.' Alternatives include 'Anti-Theft Alarm Serial Link Fault' (Land Rover), 'Valve for Pump - Jet: Cylinder 3 (N242): Electrical Malfunction' (VW/Audi), and 'Generator L-Terminal Control Circuit Malfunction' (General Motors).

Can I Drive With P1668?

No — Do Not Drive. This code causes a no-start condition or sudden stalling. Do not drive the vehicle; the engine will shut off without warning, creating a serious safety risk in traffic.

Common Causes

• Failed Injector Driver Module (IDM) or Fuel Injection Control Module (FICM) (Very Common) — The internal electronics of the module, specifically the power supply, fail from heat and vibration, preventing injector operation. This is the leading cause on Ford Powerstroke engines.
• Damaged Wiring or Connectors (Common) — The wiring harness between the PCM and IDM/FICM chafes, melts, or corrodes. This happens frequently where wires rub against vibrating valve covers or exhaust components.
• Failed IDM/FICM Power Relay or Blown Fuse (Common) — The IDM/FICM requires a dedicated relay and fuse. A blown fuse or dead relay cuts power entirely, severing PCM communication instantly.
• Internal Injector or Wiring Failure (VW/Audi) (Common) — On VW/Audi PD diesels, the injector wiring harness under the valve cover becomes brittle from hot oil, breaking the electrical connection to cylinder 3.
• Low or Unstable Battery Voltage (Less Common) — Weak batteries or a failing alternator cause severe voltage drops during cranking, disrupting digital communication between modules.
• Poor Module or Chassis Ground (Less Common) — A corroded or broken ground strap removes the stable electrical reference point, corrupting digital signals and triggering the code.
• Aftermarket Performance Tuner/Chip Malfunction (Less Common) — Piggyback performance chips alter PCM signals and corrupt the communication link. Removing the chip is a mandatory first diagnostic step.
• Failed Powertrain Control Module (PCM) (Rare) — The PCM itself fails internally, though this is significantly rarer than IDM/FICM or wiring failures.

Symptoms

• Engine Cranks But Will Not Start — The starter turns the engine, but it refuses to fire because the injectors receive no fuel delivery commands.
• Engine Stalls While Driving — The engine shuts off instantly and without warning when the communication link drops.
• Rough Running or Severe Misfiring — Intermittent communication causes the engine to shudder and misfire as injectors fire inconsistently.
• Check Engine Light On — The PCM illuminates the Malfunction Indicator Lamp (MIL) immediately upon detecting the communication fault.
• Reduced Engine Power / Limp Mode — The vehicle enters a restricted power state to protect the powertrain from erratic fueling.
• Failed Injector "Buzz Test" (scan-tool only — no driver-felt sign) — During a scan tool-commanded injector buzz test, the injectors remain silent, confirming the IDM is offline.

Diagnostic Flowchart

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

Common Fixes & Costs

• Replace the IDM/FICM Power Relay — Parts: $15-$40, Labor: $0-$25, ~0.1 hr book time (DIY)
  Ford Powerstroke (universal): OEM F8OZ-14N089-AA (Alt: Motorcraft F8OZ-14N089-AA, Standard Motor Products RY612)
• Repair Damaged Wiring or Connectors — Parts: $10-$100, Labor: $150-$500, ~3.5 hr book time (Professional)
• Replace the IDM/FICM — Parts: $250-$1200, Labor: $150-$300, ~2 hr book time (Intermediate)
  Ford 7.3L Powerstroke (1999-2003): OEM XC3Z-12B599-AA, HC2Z-12B599-ARM (Alt: Alliant Power AP65120, GB Remanufacturing 921-120)
  Ford 6.0L Powerstroke (2003-2007): OEM HC3Z-12B599-ARM (Alt: Alliant Power AP65122, Dorman 599-106)
• Repair the FICM Power Supply — Parts: $100-$400, Labor: $100-$200, ~2 hr book time (Intermediate)
• Replace Under-Valve-Cover Injector Harness (VW) — Parts: $100-$250, Labor: $200-$500, ~3 hr book time (Intermediate)
  VW Jetta/Golf TDI (BEW/BRM): OEM 038971600 (Alt: Febi 34846, Bosch 1987547001)
• BCU Reprogramming or Replacement (Land Rover) — Parts: $100-$1200, Labor: $200-$400, ~2.5 hr book time (Professional)

DIY vs Professional

• Replace the IDM/FICM — Beginner:
  Tools: Socket set, Torx driver set, flashlight.
• Repair the FICM Power Supply — Beginner:
  Tools: Torx drivers (T10, T20), pry tools, soldering iron.
• Repair Damaged Wiring — Beginner:
  Tools: Multimeter, wire strippers, soldering iron, heat shrink tubing, wiring diagrams.
• Replace the IDM/FICM Power Relay — Beginner:
  Tools: Relay puller.
• Replace VW Injector Harness — Beginner:
  Tools: Socket set, torque wrench, gasket scraper, new valve cover gasket.

Used vs. New Parts: Buying Guide

When a used part is worth it: A used OEM Ford IDM from a low-mileage donor is often more reliable than a cheap aftermarket rebuild. This is ideal for older trucks on a tight budget.

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

Donor quality checklist:

• Verify exact part number match.
• Source from a reputable yard with donor history.
• Reject parts with case corrosion or water intrusion.
• Avoid suspiciously cheap online marketplace units.

Decision logic:

• If The vehicle is a daily driver or critical for work → Buy a premium remanufactured unit with a lifetime warranty.
• If Vehicle is older (>150k miles) and budget is tight → Buy a used OEM part and accept the shorter lifespan.
• If The problem is a 6.0L FICM power supply failure → Use a mail-in repair service to upgrade the power board for under $350.

Warranty tradeoff: Salvage yards offer 30 days; premium rebuilders offer 1-year to lifetime warranties.

Worst-case if a used part fails: $300-$600 to buy a second replacement part and pay labor again.

What Happens If You Wait — Timeline

1. 0-4 weeks: Intermittent fault. The truck experiences slightly longer cold cranks or a single unexpected stall. (MPG impact: 0%% · Added cost: $0)
2. 1-3 months: Cold starts become consistently difficult. The engine stalls over large bumps. The vehicle is dangerously unreliable. (MPG impact: 0-5%% · Added cost: $100-$300 in a tow bill when the vehicle finally stalls and won't restart.)
3. 3-6 months: Hard crank-no-start. Repeated cranking overheats and destroys the starter motor and drains batteries. (MPG impact: 100% (vehicle is undrivable)% · Added cost: $400-$800 for a new starter and/or batteries.)
4. 6+ months: Catastrophic failure. The unstable voltage permanently burns out the FICM logic board, requiring a $1,200 replacement. (MPG impact: 100% (vehicle is undrivable)% · Added cost: $1200-$2000+ for a new module, starter, and batteries.)

Cost of Not Fixing It

• Immediate: Sudden stalling creates a severe crash risk in traffic. (Added cost: $100-$300 for a tow truck when the vehicle stalls in an unsafe location.)
• 1-3 months: Repeated cranking destroys the starter motor and drains batteries beyond recovery. (Added cost: $400-$800 for new batteries and a starter motor.)
• 3+ months: A failing power supply permanently burns out the FICM logic board, requiring full module replacement instead of a cheaper repair. (Added cost: $1200+ for a new module and consequential damage.)

Diagnosis Steps

1. Inspect Fuses and Relays
   Locate the IDM/FICM and PCM fuses and power relays in the under-hood fuse box. Swap the IDM relay with a known-good relay (like the horn) to instantly test for a relay failure.
   Tools: Owner's Manual, Fuse Puller (Beginner)
2. Check the Battery and Charging System
   Verify the battery holds 12.4-12.6V with the engine off and 13.7-14.7V while running. Low voltage directly causes module communication drops.
   Tools: Digital Multimeter (Beginner)
3. Perform a Visual Inspection of Wiring
   Inspect the wiring harness between the PCM and IDM/FICM. Look for chafing on valve covers, melting near the exhaust, or corrosion at the main 42-pin connector.
   Tools: Flashlight, Mirror (Intermediate)
4. Test 6.0L FICM Output Voltage
   Monitor the FICM_MPWR PID with a scan tool. Voltage must be 48V key-on and never drop below 45V during cranking. Any drop below 45V confirms a dead FICM power supply.
   Tools: Advanced Scan Tool (Advanced)
5. Test 7.3L Injector Circuit Resistance at IDM
   Disconnect the IDM and measure resistance between each injector power feed and ground pin at the harness. Readings must be 2.8-3.6 Ohms. Infinite resistance to chassis ground is required.
   Tools: Digital Multimeter, Vehicle-Specific Wiring Diagram (Advanced)
6. Test VW PD Injector Circuit Resistance
   Disconnect the under-valve-cover harness and measure the cylinder 3 injector coil resistance. It must be 0.3-0.7 ohms. If normal, the harness is faulty.
   Tools: Digital Multimeter, Basic Hand Tools (Advanced)
7. [PRO TIP] Load Test Power and Ground Circuits
   Use a headlamp bulb to load the power and ground pins on the IDM/FICM connector. A bright light confirms a healthy circuit; a dim light exposes high resistance.
   Tools: Headlamp Bulb with Wires, Power Probe, Wiring Diagram (Advanced)
8. Test Communication Wires for Continuity
   With PCM and IDM disconnected, check the communication wires for near 0 ohms continuity and ensure infinite resistance to ground.
   Tools: Digital Multimeter, Vehicle-Specific Wiring Diagram (Advanced)
9. Check for FICM Sync (Ford 6.0L)
   Monitor the 'FICM_SYNC' PID. It must read 'Yes' while cranking. 'No' indicates a loss of synchronization due to a faulty FICM logic board or wiring.
   Tools: Advanced Scan Tool (Advanced)
10. [PRO TIP] Scope Test Communication Lines
    Back-probe the communication line with an oscilloscope. A clean square wave is required. A flat or distorted line confirms a wiring short or dead module.
    Tools: Automotive Oscilloscope, Wiring Diagram (Professional)

When This Code Triggers (Freeze-Frame Conditions)

• Engine Coolant Temp: 180-205°F (Occurs when the engine is fully warmed up and components are heat-soaked, exposing intermittent electrical breaks.)
• RPM: 0 (cranking) or 1500-2500 (driving) (Logs at 0 RPM during a crank-no-start, or between 1500-2500 RPM if engine vibration severs the connection while driving.)
• Engine Load: 30-60% (Triggers under 30-60% load when increased electrical demand and vibration stress weak wiring points.)
• Vehicle Speed: 0 mph or 35-55 mph (Logs at 0 mph during starting failures or 35-55 mph during sudden highway stalls.)

Related Codes

• P1670 — Means the PCM sent a command but received no confirmation. P1668 is the primary total-failure code; fix P1668 first.
• P1316 — Indicates the IDM detected an injector fault. If P1668 is also present, the communication link is broken and must be fixed before diagnosing the injectors.
• P1298 — Directly flags an internal IDM failure. Combined with P1668, it guarantees the IDM module requires replacement.
• P1663 — Isolates the fault to the Fuel Demand Command Signal wire. P1668 is the broader communication failure umbrella.

Climate & Environmental Factors

• Cold Weather: Cold, thick engine oil requires maximum electrical energy to fire injectors. A weak 6.0L FICM drops below 45V during cold starts, causing a crank-no-start.
• Humidity and Water: Degraded case seals on the 7.3L IDM allow water intrusion. Moisture corrodes the internal circuit board, permanently killing the module.

How to Talk to a Mechanic About This Code

Say this: "I have a crank-no-start with code P1668. Before quoting a new module, please test the power relay, load-test the grounds, and inspect the valve cover wiring harness for chafing."

Directs the shop to check the $20 failure points before jumping to a $1,000 module replacement.

Avoid saying:

• 'My truck won't start, can you fix it?'
• 'The check engine light is on.'
• 'Just replace whatever it needs.'

Questions to ask before authorizing the repair:

• What were the IDM relay test results?
• Did you load-test the power circuits?
• What was the FICM voltage during cranking?
• Can you show me where the wiring harness was chafed or damaged?
• If you are recommending a module replacement, what specific tests confirmed the old module was faulty and that the wiring is good?

Where to Take It: Dealer vs Independent vs Chain

• Dealer: Best for Land Rover/BMW anti-theft resyncs. Avoid for older Ford trucks due to high labor rates.
  Best for: Vehicles under warranty., Land Rover, Jaguar, or BMW anti-theft issues that require proprietary software to reprogram or sync modules.
  Downsides: Significantly higher labor rates., May be less willing to diagnose complex wiring issues and more likely to recommend expensive module replacement. (Typical cost: +50% vs. baseline)
• Independent Shop: The best choice for Ford Powerstroke and VW TDI issues. Diesel specialists fix this exact code weekly.
  Best for: Out-of-warranty Ford Powerstroke trucks., VW TDI injector harness issues., Getting cost-effective diagnosis of wiring and electrical systems.
  Downsides: Quality and experience vary greatly. Look for a shop that specializes in diesel trucks., May lack the expensive, specialized tools for reprogramming anti-theft modules on some European brands. (Typical cost: +0% vs. baseline)
• Chain Shop: AVOID. Lacks the specialized scanners and diesel expertise required, leading to expensive misdiagnoses.
  Best for: Simple maintenance like oil changes and tires.
  Downsides: Technician skill is highly variable and often not specialized for complex diesel or electrical diagnostics., High pressure to sell parts can lead to misdiagnosis and unnecessary replacement of expensive components. (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the repair exceeds 50% of the vehicle's value, reconsider. For critical work trucks, this threshold extends to 75%.

• Car worth $8000, fix is $1800: Fix it. A $1,800 repair for a new FICM and labor on an $8,000 truck is well within a reasonable threshold.
• Car worth $4500, fix is $1500: Borderline. The repair is one-third of the truck's value. Get a second opinion to confirm the diagnosis before proceeding.
• Car worth $3000, fix is $2000: Walk away. The repair cost is too high relative to the vehicle's value. You are unlikely to recoup this investment.

What Scan Tool You Need for This Code

Minimum: Must read manufacturer-specific PIDs (like FICM Main Power) and perform bidirectional controls (like an Injector Buzz Test).

A $20 reader only shows 'P1668'. It cannot read the live voltage data required to prove the module is dead.

Budget: FORScan with ELM327 Adapter (~$40) — Provides dealer-level Ford diagnostics, reads FICM voltage, and runs buzz tests.

Mid-range: Autel MX808 / Foxwell NT510 Elite (~$250-400) — Handheld scanners with bidirectional controls for buzz tests.

Professional: Autel MaxiSys MS906BT / XTOOL D7 or D8 (~$600-1200) — Professional tools capable of Land Rover BCU resyncs and full diesel diagnostics.

Rent vs buy: For this specific code, the free rental scanners from auto parts stores are often too basic and will not have the required bidirectional or manufacturer-specific functions. If you own a Powerstroke or do regular DIY work, investing in FORScan or a midrange scanner is highly recommended and will pay for itself quickly.

How to Clear the Code After You Fix It

1. Reconnect the battery if it was disconnected during repair.
2. Erase the DTCs from the PCM memory using an OBD-II scan tool.
3. Start the engine and idle for 5 minutes to verify the immediate fault is gone.
4. Complete a full drive cycle to reset emissions readiness monitors.

Drive cycle (~30 minutes): A comprehensive drive cycle involves: 1) A cold start. 2) 5-10 minutes of idling. 3) 15-20 minutes of mixed city driving with stops. 4) 5-10 minutes of steady highway speed (55-65 mph). 5) Full cool down.

Readiness monitors affected: Clearing any DTC resets all emissions readiness monitors, including Catalyst, Evaporative System, Oxygen Sensor, and EGR monitors.

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

Watch out for:

• Disconnecting the battery clears the code but resets all readiness monitors; the code returns instantly if the hardware fault remains.
• Testing before monitors read 'Ready' guarantees an emissions failure.

Will This Fail Emissions / State Inspection?

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

• California: An active P1668 instantly fails the OBD-II check. A full drive cycle is mandatory after repair.
• New York: The active code fails the emissions portion of the NYS safety/emissions inspection.
• Texas: Vehicles in emissions counties fail the OBD-II plug-in test with an active P1668.

Most Commonly Affected Vehicles

• Ford F-250/F-350 Super Duty (7.3L Powerstroke) (1999-2003) — Extremely common due to IDM water intrusion and wiring harness chafing on the driver-side valve cover.
• Ford F-250/F-350 Super Duty (6.0L Powerstroke) (2003-2007) — FICM power supplies fail consistently from heat and vibration, especially during cold starts.
• Ford Excursion (6.0L & 7.3L Powerstroke) (2000-2005) — Shares the exact IDM/FICM failure rates as Super Duty trucks.
• Land Rover Discovery II (1999-2004) — Triggers when the BCU and ECM lose anti-theft synchronization, requiring dealer reprogramming.
• Jaguar S-Type, XJ8, XK8 (1999-2005) — Flags an immobilizer communication fault preventing engine start.
• Volkswagen Golf, Jetta, Passat (TDI with PD engines) (2002-2006) — Specifically indicates a failed cylinder 3 injector wiring harness under the valve cover.
• BMW Various Models (e.g., E46, E90) (2000-2015) — Indicates a corrupted start value signal between the EWS immobilizer and the DME.
• Chevrolet / GMC HHR, Cobalt, Colorado, various trucks (2005-2015) — Means 'Generator L-Terminal Control Circuit Malfunction'—strictly an alternator issue.

Manufacturer-Specific Notes

• Ford: A failed injector buzz test definitively proves the IDM/FICM or its wiring is at fault.
• Jaguar / Land Rover: Locking and unlocking the doors with the key fob sometimes temporarily bypasses this anti-theft lockout.
• General Motors (Chevrolet/GMC): P1668 strictly flags an alternator turn-on signal failure, completely unrelated to fuel or anti-theft systems.
• Volkswagen / Audi: A 'wiggle test' on the under-valve-cover harness while the engine idles instantly confirms the cylinder 3 short.

Real Owner Stories

2002 Ford F-250 7.3L at 210K miles - The Misdiagnosis

Truck started sputtering on the highway, then stalled. Would crank but not start. Intermittently, the scanner could not communicate with the truck at all.

What they tried:

1. Initially suspected bad fuel.
2. Replaced the Exhaust Back Pressure (EBP) sensor based on a P0472 code.
3. Considered replacing the IDM immediately.

Outcome: The owner found the wiring harness rubbed through on the driver's side valve cover. Repairing the wires restored communication.

Lesson: Never replace an expensive module first. A visual inspection of the harness is a mandatory, free diagnostic step.

2005 Ford F-350 6.0L at 165K miles - The Cold Weather Killer

Truck became very hard to start in the morning, especially on cold days. Eventually, it became a crank-no-start.

What they tried:

1. Checked batteries and alternator, both were good.
2. Performed a FICM voltage test using a scan tool.
3. Observed FICM voltage was 48V with key on, but dropped to 35V during cranking.

Outcome: Scan tool showed FICM voltage dropping to 35V during cranking. A $350 mail-in repair fixed the cold-start issue.

Lesson: A scan tool voltage test definitively proves a failing 6.0L FICM. Mail-in repairs save hundreds over buying new.

2004 VW Jetta TDI (BEW) at 140K miles - The Specific Culprit

Check Engine Light came on, and the engine had a noticeable misfire/stumble at idle. The code was P1668.

What they tried:

1. Owner suspected a bad injector, which would be an expensive repair.
2. A mechanic suggested checking the injector wiring harness first.
3. While the engine was running, the mechanic wiggled the injector harness connector and was able to make the misfire come and go.

Outcome: A wiggle test on the injector harness caused the misfire to toggle. Replacing the $150 harness fixed the P1668.

Lesson: On VW PD engines, P1668 almost always points to the under-valve-cover harness, not the expensive injector itself.

2003 Land Rover Discovery II - The Anti-Theft Gremlin

Truck died while pulling into a parking lot and then would only crank but not start. The code reader showed P1668.

What they tried:

1. Checked battery voltage, which was slightly low.
2. Tried locking and unlocking the doors with the key fob.
3. Researched forums and found a Technical Service Bulletin (TSB) from 2001.

Outcome: The BCU and ECM lost synchronization. Fully charging the battery and locking/unlocking the doors temporarily bypassed the immobilizer.

Lesson: Land Rover P1668 is a security fault. Ensure perfect battery health before paying a dealer to resync the modules.

How to Prevent This Code From Triggering

• Protect the 7.3L IDM from Water (Once) — Apply a U-shaped bead of silicone around the IDM case vent to deflect water without sealing it shut.
• Secure and Protect Wiring Harnesses (During any under-hood maintenance) — Zip-tie loose harnesses and wrap vulnerable sections in high-temp tape to prevent valve cover chafing.
• Maintain Healthy Battery Voltage (Ongoing) — Test batteries annually and replace in pairs. Low voltage forces the FICM to overheat and fail prematurely.
• Upgrade the 6.0L FICM Power Supply Proactively (Once, especially on high-mileage or critical-use trucks) — Install an upgraded aftermarket FICM power supply proactively to prevent being stranded by a cold-start failure.
• Check and Clean Chassis/Module Grounds (Every 30,000 miles) — Unbolt, wire-brush, and tighten chassis ground straps every 30,000 miles to ensure stable module voltage.

Frequently Asked Questions

Can I just replace the fuse for the IDM to fix P1668?

Replacing a blown IDM fuse is a temporary fix. Fuses blow due to wiring shorts or a failing module drawing excessive current; the new fuse will blow again.

What is the most common misdiagnosis for P1668 on a Powerstroke?

The most common mistake is replacing a $500 module without testing the $20 power relay. Always swap the IDM relay with the horn relay first.

Can my aftermarket performance chip cause a P1668 code?

Yes. Piggyback tuners intercept and corrupt PCM signals. Remove the tuner completely to see if the P1668 code clears.

What is an IDM or FICM?

The IDM/FICM amplifies the PCM's low-voltage signal into the high voltage (48V-115V) required to fire diesel injectors.

Is it better to repair my FICM or buy a new one?

Mail-in repair services ($100-$400) upgrade the weak factory components and are highly reliable. Remanufactured units are faster but cost up to $1,200.

My truck stalled but then restarted. Is it safe to drive?

No. An intermittent P1668 means the engine will shut off randomly. Driving it risks a sudden stall in high-speed traffic.

Why did P1668 appear after my batteries died?

Dead batteries cause severe voltage drops during jump-starts, corrupting module communication. Fully charge the batteries and clear the code to see if it returns.

What is the difference between P1668 and P1670?

P1668 means the PCM cannot send a signal to the IDM at all. P1670 means the PCM sent the signal but the IDM failed to reply.

Can a bad Camshaft Position Sensor (CPS) cause a P1668?

No. A bad CPS causes a no-start and logs P0344, but it does not cause a P1668 communication error. Fix the P1668 fault first.

Key Takeaways

• P1668 indicates a total communication failure between the main engine computer (PCM) and the fuel injector module (IDM/FICM).
• On 1999-2007 Ford Powerstroke diesels, this code guarantees a crank-no-start or sudden stalling condition.
• Never replace a $500+ IDM or FICM without first testing the $20 power relay and inspecting the valve cover wiring harness for chafing.
• For Land Rover and Jaguar owners, P1668 signals an anti-theft immobilizer lockout requiring a battery reset or dealer-level module resynchronization.
• Volkswagen TDI models trigger P1668 for a failed cylinder 3 injector harness, while GM vehicles use it to flag a faulty alternator control circuit.