OBD-II Code P0688: ECM/PCM Power Relay Sense Circuit / Open

What Does P0688 Mean?

P0688 signifies the car's main computer, the Powertrain Control Module (PCM), detected a fault in its own power supply. The module uses a 'sense' circuit to monitor voltage from the main power relay. When the PCM commands the relay on, it expects battery voltage on this sense circuit. If it sees incorrect or zero voltage, it logs P0688. Think of it as the computer performing a self-check on its power source and finding a critical error.

Technical definition: The official SAE/OBD-II definition is "ECM/PCM Power Relay Sense Circuit / Open". When the PCM energizes the main relay by grounding the control coil, it expects near-battery voltage on a separate sense wire. If this voltage is absent or drops below a manufacturer-specified threshold (e.g., 9.0 volts), the PCM detects an open circuit and sets the code.

Can I Drive With P0688?

No — Do Not Drive. This code causes a no-start condition or makes the engine stall without warning while driving. It represents a critical failure in the engine computer's power supply. Driving is not recommended as the vehicle shuts down unpredictably, posing a significant safety risk.

Common Causes

• Blown Fuse or Fusible Link (Very Common) — A blown fuse is the top cause. This fuse protects the PCM power relay circuit. On Dodge, Ram, and Jeep vehicles, the ignition coil fuse is shared with the relay sense circuit, making it the primary suspect.
• Shorted Ignition Coil (Chrysler/Dodge/Jeep/Ram) (Very Common) — On Chrysler-family vehicles, a cracked or internally shorted ignition coil blows the shared fuse for the ignition and ASD (Auto Shutdown) relay sense circuit, directly causing a P0688 code even if the relay is perfectly functional.
• Faulty PCM/Main Power Relay (Very Common) — Relays are mechanical switches that wear out. They fail internally, get stuck, or develop burnt contacts that prevent power from passing through, leading to a loss of voltage on the sense circuit.
• Wiring or Connector Issues (Common) — Damaged, corroded, or loose wiring between the battery, fuse box, relay, and PCM creates an open or high-resistance circuit. This includes the main power and ground wires, the sense wire, and the relay socket pins.
• Corrosion in Fuse Box (TIPM/BJB) (Less Common) — The main fuse box (TIPM in Chrysler, BJB in Ford) suffers from water intrusion. This corrodes internal circuits and relay connections, causing high resistance and triggering the fault.
• Weak or Dead Battery / Poor Battery Connections (Less Common) — A battery with low voltage (below 12.4V) or corroded terminals cannot provide stable power. The PCM misinterprets this low voltage as a relay failure, triggering a false P0688 code.
• Failing Alternator or Voltage Regulator (Less Common) — A faulty alternator produces unstable voltage, including spikes or drops. The PCM monitors system voltage and interprets this erratic power supply on the sense circuit as a relay failure.
• Faulty Powertrain Control Module (PCM) (Rare) — An internal PCM failure, such as a cracked solder joint or failed driver circuit, triggers P0688. Exhaustively rule out all other possibilities before considering a $500+ PCM replacement.

Symptoms

• Engine will not start (cranks but won't fire) — The starter turns the engine over, but the unpowered PCM cannot command the fuel injectors and ignition coils to operate.
• Engine stalls intermittently while driving — The vehicle runs normally and suddenly shuts off. This occurs when the PCM power relay or circuit fails momentarily, cutting power to the engine's brain.
• Check Engine Light is on — The PCM stores the P0688 code and illuminates the Malfunction Indicator Lamp (MIL).
• Delayed engine start — The engine cranks for an extended period before starting, caused by an intermittent relay or high-resistance connection that eventually allows enough voltage to pass.
• Accessory electrical anomalies — Because the PCM is a central network node, its failure causes strange behavior in other systems, such as flickering dash lights or erratic gauge readings.
• No communication with scan tool (scan-tool only — no driver-felt sign) — Because the PCM is not powering on, a diagnostic scan tool cannot establish a connection. This confirms the PCM's power supply is the root problem.

Diagnostic Flowchart

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

Common Fixes & Costs

• Replace Blown Fuse(s) — Parts: $1-$10, Labor: $0-$50, ~0.2 hr book time (DIY)
• Replace Shorted Ignition Coil — Parts: $40-$100 per coil, Labor: $50-$150, ~0.8 hr book time (DIY)
• Replace PCM/Main Power Relay — Parts: $15-$50, Labor: $25-$100, ~0.3 hr book time (DIY)
• Repair Damaged Wiring or Connectors — Parts: $10-$50, Labor: $150-$400+, ~2.5 hr book time (Intermediate)
• Replace Powertrain Control Module (PCM) — Parts: $500-$1,400+, Labor: $150-$300, ~1.5 hr book time (Professional)

Used vs. New Parts: Buying Guide

When a used part is worth it: For relays and ignition coils, new aftermarket parts offer the best reliability. A used PCM is only viable if it matches the exact OEM part number and you have a plan for professional reprogramming.

Donor-vehicle mileage cap: roughly under 60000 miles for the part to have meaningful remaining life.

Donor quality checklist:

• Buy new ignition coils; used parts have unknown remaining life.
• Match the PCM OEM part number exactly. Verify the donor vehicle lacked electrical or flood damage.
• Ensure used parts include a 30-day warranty.

Decision logic:

• If The part is a relay, fuse, or ignition coil → Buy a new part from a reputable aftermarket or OEM supplier. Cost is low and reliability is critical.
• If The part is a PCM and the vehicle is over 10 years old → A pre-programmed, remanufactured PCM from a specialized vendor is better than a junkyard part.
• If The part is a wiring harness or connector → Used pigtails are viable for repairing specific connectors. Ensure zero corrosion.

Warranty tradeoff: New aftermarket parts include 1-year to lifetime warranties. Salvage yard parts offer 30-90 days. A failed used part costs you the part price plus repeat labor.

Worst-case if a used part fails: $300-$1500 if a used PCM fails after programming or an electrical short damages other components.

What Happens If You Wait — Timeline

1. Immediate: The fault occurs, setting code P0688. The vehicle experiences a 'crank, no start' condition or stalls while driving, rendering it disabled and unsafe. (MPG impact: N/A (vehicle won't run)% · Added cost: $100-$300 for a tow.)
2. Days to Weeks (if intermittent): Intermittent faults cause random stalling. Each abrupt power loss risks software corruption or damage to sensitive internal PCM components. (MPG impact: 0-5% (due to inefficient operation during fault events)% · Added cost: $0-$50 in wasted fuel, plus continued safety risk.)
3. Weeks to Months: Repeatedly cranking against a dead short overheats fuse terminals within the fuse box (TIPM/BJB), escalating a simple fuse replacement into a complex wiring repair. (MPG impact: N/A% · Added cost: $250-$1000+ for fuse box repair or replacement.)
4. Months+: Over-voltage from a faulty alternator permanently damages the PCM. Intermittent power supplies lead to internal PCM failure from thermal stress. (MPG impact: N/A% · Added cost: $800-$2000+ for a replacement PCM and module programming.)

Cost of Not Fixing It

• Immediate: Vehicle is unreliable, will not start, or stalls in traffic, posing a significant safety risk. (Added cost: $100-$300 for a tow.)
• 1-4 Weeks: Repeatedly trying to start the vehicle with an underlying short damages fuse box terminals. Intermittent power to the PCM corrupts its software. (Added cost: $200-$800 for fuse box or wiring repair.)
• 1+ Month: Excessive voltage from a bad alternator permanently damages the PCM and other sensitive electronic modules. (Added cost: $800-$2000+ for PCM and module replacement.)

Diagnosis Steps

1. Verify the Code and Check Freeze Frame Data
   Confirm P0688 is present. Check for misfire codes (P0300-P0308) or P0685, which point to the root cause. Analyze freeze frame data to see vehicle conditions when the code set.
   Tools: ['OBD-II Scanner']
2. Thoroughly Inspect All Related Fuses
   Locate the fuse box and inspect fuses related to the PCM, fuel, and ignition. On Ram/Jeep vehicles, test ignition coil fuses (e.g., F63, F90, M19). Use a multimeter to test for continuity; do not rely on visual inspection.
   Tools: ['Multimeter', 'Fuse Puller']
3. Locate and Swap the PCM/Main Power Relay
   Find the PCM power relay (labeled "PCM," "MAIN," or "ASD"). Swap it with an identical relay from a non-essential system (like the horn). If the vehicle starts, the original relay is faulty.
   Tools: ['Multimeter']
4. Inspect the Battery and Connections
   Ensure the battery is fully charged (12.4-12.6V engine off) and terminals are clean and tight. A healthy charging system shows 13.5-14.8V with the engine running.
   Tools: ['Multimeter', 'Wire Brush', 'Wrench Set']
5. Test for Power and Ground at the Relay Socket
   Check the relay socket terminals. You need constant battery voltage (pin 30), ignition-switched power (pin 86), and a ground signal from the PCM (pin 85). This confirms power reaches the relay and the PCM controls it.
   Tools: ['Multimeter', 'Vehicle-Specific Wiring Diagram']
6. Perform a Relay Voltage Drop Test
   With the relay installed and commanded ON, measure voltage between the power input pin (30) and output pin (87). A drop greater than 0.1 volts indicates high internal resistance, requiring relay replacement.
   Tools: ['Multimeter', 'Jumper Wires']
7. Test Relay Coil Resistance
   Remove the relay and measure resistance between the control circuit pins (85 and 86). Typical resistance is 50-200 Ohms. Infinite resistance (OL) means a broken coil; near zero means a shorted coil.
   Tools: ['Multimeter']
8. Check the Sense Circuit for an Open or Short
   Disconnect the PCM and relay. Check continuity on the sense wire from the relay socket (pin 87) to the PCM connector. Infinite resistance (OL) indicates a broken wire. Check resistance to ground; any continuity indicates a short.
   Tools: ['Multimeter', 'Vehicle-Specific Wiring Diagram']
9. Check Live Data PIDs
   Monitor 'PCM Voltage' or 'Module Input Voltage'. It must match battery voltage. Erratic readings or voltage below 10V confirms a power supply issue.
   Tools: ['Advanced OBD-II Scan Tool'] (Advanced)
10. PCM Sense Circuit Voltage Test
    With the key on, back-probe the PCM power relay sense circuit wire at the PCM connector. Voltage must be greater than 9.0V. Lower readings point to high circuit resistance or a failing relay.
    Tools: ['Digital Multimeter', 'Back-probe Kit', 'Vehicle-Specific Wiring Diagram'] (Advanced)

When This Code Triggers (Freeze-Frame Conditions)

• Fault Trigger: Key On or Cranking (The fault is detected immediately when the ignition turns on or during engine cranking, as the PCM performs its power-on self-test.)
• RPM: 0 or > 1500 (The code sets at 0 RPM (crank-no-start) or during steady-state driving if the relay or wiring fails intermittently, causing a stall.)
• Module Voltage: < 9.0V (Freeze frame data shows the PCM voltage reading significantly below normal battery voltage, which directly triggers the code.)
• Engine Status: Not Running or Stalling (The condition is associated with the engine failing to start or cutting out entirely.)

Related Codes

• P0685 — P0685 points to a fault in the low-current control side of the relay. P0688 points to the high-current load/sense side. If both are present, diagnose P0685 first.
• P0300-P0308 — Misfire codes are a major clue. A shorted ignition coil causes a misfire and blows the fuse shared with the PCM relay sense circuit. Investigate the misfiring cylinder's coil immediately.
• U0100 — Lost Communication With ECM/PCM. If the PCM lacks proper power due to the P0688 fault, it cannot communicate on the vehicle's network. Resolving P0688 resolves U0100.
• P0562 — System Voltage Low. Accompanies P0688 if a failing battery or alternator cannot provide sufficient voltage for the electrical system.

Climate & Environmental Factors

• High Humidity / Water Intrusion: Moisture infiltrates the fuse box (TIPM/BJB) and relay sockets, causing corrosion on pins and internal circuits. This increases resistance, leading to voltage drops that trigger P0688. This is heavily documented on Ford and Chrysler vehicles.
• Extreme Cold: Cold weather reduces battery performance, dropping voltage low enough during cranking to trigger a false P0688 code. Older relays become mechanically sticky in the cold, failing to make proper contact.
• Vibration: Constant road vibration chafes wiring through its insulation, leading to intermittent shorts to ground. It causes terminals within connectors or the fuse box to lose tension, creating poor connections.

How to Talk to a Mechanic About This Code

Say this: "I have a P0688 code with a crank-no-start condition. Please test the PCM power relay, its socket, and related fuses with a multimeter before diagnosing complex modules. If this is a Chrysler/Dodge/Jeep, please check for shorted ignition coils first."

This directs the technician to perform a logical, cost-effective diagnosis instead of immediately quoting an expensive PCM or TIPM replacement.

Avoid saying:

• 'My car won't start, can you fix it?'
• 'The check engine light is on, just do whatever it takes.'
• 'I think I need a new computer.'

Questions to ask before authorizing the repair:

• Did you test the PCM relay and check for voltage drop, or just swap it?
• If a fuse was blown, did you identify the short circuit that caused it to blow?
• Can you confirm you tested for power and ground at the relay socket according to the wiring diagram?
• Before replacing the PCM, can you show me test results proving all other circuit parts are good?

Where to Take It: Dealer vs Independent vs Chain

• Dealer: Recommended only if warranty applies or if an independent shop definitively condemned a programmable module.
  Best for: Vehicles under powertrain warranty., Confirmed failures of a TIPM or integrated module requiring manufacturer-specific programming.
  Downsides: Highest labor rates., Quick to replace entire modules rather than perform detailed wiring repairs. (Typical cost: +50% vs. baseline)
• Independent Shop: Best fit. P0688 is a classic electrical problem a skilled independent technician solves efficiently.
  Best for: Out-of-warranty vehicles., Electrical diagnostics requiring methodical testing of circuits and relays.
  Downsides: Diagnostic skill varies. Look for ASE-certified technicians specializing in electrical work. (Typical cost: +0% vs. baseline)
• Chain Shop: AVOID for initial diagnosis. The risk of misdiagnosis and unnecessary parts replacement is high.
  Best for: Simple part swaps AFTER the problem is correctly diagnosed elsewhere.
  Downsides: Technician skill varies dramatically; not ideal for complex electrical diagnosis., Business model incentivizes quick parts replacement over detailed diagnostic work. (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the estimated repair cost exceeds 50% of the car's private-party value, seriously consider not fixing it. For older, high-mileage cars, this threshold drops to 40%.

• Car worth $10000, fix is $350: Fix it. A repair for a bad relay or ignition coil is a small fraction of the car's value.
• Car worth $4000, fix is $1800: Walk away. A quote this high means a PCM or TIPM replacement. This repair cost is 45% of the car's value, making it a poor investment. Get a second opinion.
• Car worth $2000, fix is $1500: Walk away. The repair cost is 75% of the car's value. It is not financially sound to proceed.

What Scan Tool You Need for This Code

Minimum: A scanner that reads codes, clears codes, and displays live data, specifically 'Module Voltage'.

A $20 code reader only confirms the P0688 code exists. It cannot show the live voltage the PCM sees, forcing you to guess and replace parts blindly.

Budget: BlueDriver Pro (~$90) — Reads the code, provides freeze frame data, and streams live data like module voltage to your smartphone.

Mid-range: Foxwell NT510 Elite (~$180) — Provides bidirectional controls, allowing you to command the PCM power relay on and off directly from the scanner to test its function.

Professional: Autel MaxiCOM MK808 / MK808BT (~$450-550) — Offers full bidirectional control, extensive live data graphing, and quickly scans all systems to see if other modules report communication losses.

Rent vs buy: For a one-time diagnosis, auto parts stores offer free loaner tools. If you perform maintenance more than once a year, buying a scanner is a worthwhile investment.

How to Clear the Code After You Fix It

1. Fix the underlying electrical fault (e.g., replace fuse, relay, or coil).
2. Reconnect the battery if disconnected for repair.
3. Use an OBD-II scan tool to erase P0688 and related fault codes.
4. Perform a complete drive cycle to run readiness monitors.

Drive cycle (~20 minutes): Cold start and idle for 2-3 minutes. Drive in mixed city/highway traffic for 15-20 minutes at steady speeds between 40-60 mph. Idle for 2-3 minutes. Shut off the engine and let it sit for 10 minutes.

Readiness monitors affected: All monitors, Catalyst monitor, EVAP system monitor, O2 sensor monitor

Before emissions retest: drive at least 50 miles to fully set monitors.

Watch out for:

• Disconnecting the battery clears the code but resets all readiness monitors to 'Not Ready', guaranteeing an emissions test failure.
• The code returns immediately if the root electrical fault (like a shorted coil blowing a fuse) is not repaired.

Will This Fail Emissions / State Inspection?

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

• California: A P0688 code is an automatic smog check failure. A full drive cycle must be completed to set readiness monitors before a re-test.
• New York: An active P0688 code and illuminated Check Engine Light results in an immediate NYS DMV inspection failure.
• Texas: An active P0688 code is an automatic failure. After repair, you can have at most one readiness monitor 'Not Ready' to pass.

Most Commonly Affected Vehicles

• Ram 1500 (2008-2022) — Extremely common. The root cause is a shorted ignition coil blowing the fuse for the ASD relay sense circuit (check fuses F63, F90, or F50). P0688 is a symptom, not the cause. Real-world repair cost is typically $150-$250.
• Jeep Grand Cherokee, Wrangler, Cherokee (2005-2018) — Shares the Ram design. A faulty ignition coil blowing the ASD fuse (check M19 or F70) is a frequent cause of a crank-no-start with P0688. Corrosion in the TIPM is also a known issue.
• Ford F-150, Escape, Fusion (2004-2015) — Caused by a faulty PCM power relay or corrosion in the Battery Junction Box from water intrusion. On some models, a failed ignition coil damages the PCM driver.
• Dodge Durango, Charger, Stratus (2004-2014) — Identical to the Ram/Jeep ASD relay design. A shorted ignition coil or fuel injector blows the controlling fuse and logs P0688.
• Chevrolet / GMC Silverado, Sierra, Tahoe (2007-2013) — Issues trace to the underhood fuse block (UBEC). Poor connections or corrosion on the terminals for the 'RUN/CRNK' relay cause a P0688 and a no-start condition.
• Volkswagen Jetta, Passat, Golf (2001-2009) — Related to a faulty main power supply relay (labeled J271, '109', or '460'). Damaged wiring in the harness near the ECM is a known failure point.
• BMW 3-Series (E90), 5-Series (E60) (2006-2011) — Related to the main power relay (blue relay in the e-box) or failure of the Junction Box Electronics (JBE) module. A weak or unregistered battery frequently triggers this code.
• Hyundai Sonata, Elantra (2011-2016) — Caused by a faulty main relay in the engine room fuse box or wiring issues related to previous repairs or rodent damage.

Manufacturer-Specific Notes

• Dodge/Ram/Jeep (Chrysler): The Auto Shutdown (ASD) relay is the main power relay. A shorted ignition coil blows a shared fuse (e.g., F63, M19), logging P0688 even if the relay is good. Always diagnose misfire codes first.
• Ford: The PCM power relay is in the Battery Junction Box (BJB), which is susceptible to water intrusion from leaking windshield seals. Look for moisture or corrosion in the fuse box.
• Volkswagen/Audi: The main power relay is labeled 'J271', '109', or '460'. The power supply wire in the harness frequently breaks or corrodes within a foot of the ECM connector.
• General Motors (Chevrolet/GMC): The relay is labeled 'RUN/CRNK' in the underhood fuse block. Terminals in the fuse block lose tension or corrode, requiring technicians to repair or re-tension female terminals from underneath.

Real Owner Stories

2010 Dodge Ram 1500 5.7L with crank-no-start

While driving on the highway, the truck suddenly shut off and would not restart. The engine cranked but did not fire. A scan revealed code P0688.

What they tried:

1. Tested fuel delivery, which passed.
2. Swapped the ASD relay, which did not solve the issue.
3. Discovered a continually blowing 25A fuse (fuse 52).
4. Found a stored misfire code for cylinder 7.

Outcome: The ignition coil on cylinder 7 had a visible crack and shorted to the engine block, blowing the fuse. Replacing the $50 ignition coil and the $2 fuse resolved the P0688 code and the no-start condition.

Lesson: On Chrysler/Dodge/Ram vehicles, P0688 is a symptom, not the cause. If accompanied by a misfire code (P030x), the ignition coil for that cylinder is the primary suspect for blowing the shared fuse.

2014 Ram 1500 with intermittent stalling and P0688

The truck died on the road and exhibited a crank-no-start condition. A mechanic diagnosed P0688 and condemned the TIPM (fuse box) because the ASD relay behaved incorrectly during cranking.

What they tried:

1. Verified the fuel pump worked.
2. Tested the ASD relay for continuity and resistance, which passed.
3. Observed abnormal relay behavior when cranking.

Outcome: The owner sought a second opinion online to avoid a $1,200 TIPM replacement. The root cause was a shorted external component blowing the fuse that controls the relay's sense circuit, proving the initial TIPM diagnosis wrong.

Lesson: Be cautious of diagnoses that immediately condemn expensive modules like the TIPM or PCM. A blown fuse or shorted external component is a much more common and cheaper root cause.

Ford F-150 with no-start and P0688

A Ford F-150 would not start, and the P0688 code was present. The owner attempted DIY diagnosis.

What they tried:

1. Checked fuses and swapped the PCM power relay with a known good one, but the problem persisted.

Outcome: Forum advice pointed to water intrusion into the Battery Junction Box (BJB). Moisture caused corrosion on the relay pins and internal BJB circuits, creating high resistance that triggered P0688.

Lesson: On Ford vehicles, if fuses and relays test good, thoroughly inspect the fuse box (BJB) for white or green corrosion. A corroded BJB is a frequent cause of this code.

How to Prevent This Code From Triggering

• Clean Battery Terminals and Tighten Connections (Every 6 months) — Corrosion creates resistance, lowering vehicle voltage and triggering false power codes like P0688. Clean, tight connections ensure stable power.
• Inspect Fuse Box (TIPM/BJB) for Moisture (Annually, especially before winter) — Water leaks onto the main fuse box in Ford and Chrysler vehicles, causing corrosion that leads to P0688. Ensure cowl seals are intact and apply dielectric grease to connector seals.
• Protect Engine Bay Wiring (As needed during other maintenance) — Heat and vibration chafe wiring insulation, leading to shorts. Replace broken plastic conduit with new split loom and secure it with high-temp harness tape.
• Use a Battery Tender for Stored Vehicles (Whenever vehicle is stored for more than 2-3 weeks) — A battery tender prevents low-voltage situations that cause false codes upon startup and extends overall battery life.
• Avoid Overloading Electrical Circuits (Ongoing) — Adding high-draw aftermarket accessories without dedicated relays overloads factory circuits, blowing fuses and damaging wiring.

Frequently Asked Questions

Can I just replace the PCM to fix P0688?

No. This is the most expensive and least likely solution. Always diagnose and rule out cheaper causes like fuses, relays, and wiring first. Replacing a PCM unnecessarily wastes $500 to $1,400.

What are the most common misdiagnosis mistakes with P0688?

The top mistakes are replacing the PCM without testing the circuit, and visually checking a fuse instead of using a multimeter. Other errors include replacing a relay when the socket is corroded, or ignoring a shorted ignition coil on Chrysler vehicles.

My car cranks but won't start. Is P0688 the cause?

Yes, it is a primary cause. P0688 indicates the engine computer lacks the power required to operate the fuel and ignition systems. This results in a classic 'crank, no start' symptom.

I replaced the fuse and it blew again immediately. What now?

A fuse that blows immediately indicates a dead short to ground. On Ram and Jeep vehicles, this is almost always caused by a faulty ignition coil. Disconnect all coils, replace the fuse, and reconnect them one by one until the fuse blows to identify the culprit.

What is an ASD relay?

ASD stands for Auto Shutdown, the name Chrysler uses for the main power relay supplying voltage to the PCM, fuel injectors, and ignition coils. A fault in the ASD circuit prevents the engine from running.

Why does a bad ignition coil cause a PCM power code?

In many designs, the same fuse powers both the ignition coils and the PCM's power relay sense circuit. When a coil shorts, it draws excessive current and blows the shared fuse. The PCM loses voltage on its sense line and incorrectly reports a relay problem.

Why is it unsafe to drive with a P0688 code?

The fault lies in the main power supply to the engine's computer. This connection fails unpredictably, causing the engine to stall completely while driving. This leads to a sudden loss of power steering and brakes, creating a serious safety hazard.

Key Takeaways

• Code P0688 indicates a critical loss of power to the engine computer, resulting in an immediate no-start condition or dangerous stalling at highway speeds.
• Always test the PCM power relay and its dedicated fuses first, as these $10 to $30 components resolve over 70% of P0688 cases.
• On Dodge, Ram, and Jeep vehicles, a shorted ignition coil frequently blows the shared Auto Shutdown (ASD) fuse; inspect coils for cracks if misfire codes accompany P0688.
• Never replace the Powertrain Control Module (PCM)—a $500 to $1,400 repair—without first performing a voltage drop test on the relay circuit to prove the wiring is intact.