How to Fix Diagnostic Code P1664

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šŸšØ Diagnostic Profile

Vehicle Make: Chevrolet (CHEVY)

Description: Chevrolet vehicle reporting a fault in the 1-4 Upshift Lamp Control Circuit. This circuit typically controls the illumination of the shift indicator lamp that signals the driver when to upshift for optimal fuel economy, and is commonly associated with the transmission control module (TCM) or powertrain control module (PCM) and the instrument cluster or shift indicator interface. On most GM applications with this fault (commonly logged as P1890 or similar legacy manual-transmission upshift lamp codes on older Chevrolet trucks and vans), the PCM or TCM grounds or sources voltage to the upshift lamp driver circuit, which runs directly to the instrument cluster indicator or a dash-mounted lamp. The fault is set when the module detects an open, short-to-ground, or short-to-voltage condition on this control circuit, or when the commanded state does not match the sensed feedback state.

šŸšØ Repair Difficulty Tier

Tier: [Intermediate]

Diagnosis requires scan tool access, reading of live data, and wiring continuity checks. Repair is often a connector, wiring, or software issue, but may require module programming or replacement, which is beyond basic DIY scope.

šŸ› ļø Resolution Steps

Step 1: Preliminary Inspection

  • Verify battery voltage is within the normal resting range (approximately 12.4Vā€“12.6V key-off) and confirm charging system output falls within normal alternator charging range (approximately 13.5Vā€“14.7V) at idle with accessories off
  • Check for active and stored diagnostic trouble codes across all modules (PCM, TCM, instrument cluster, BCM) and record freeze frame data for the upshift lamp circuit fault
  • Inspect the instrument cluster and shift indicator lamp for burnt bulbs, damaged sockets, or corrosion at the bulb holder contacts
  • Inspect connectors at the instrument cluster, TCM/PCM, and the shift indicator circuit splice/connector for pin damage, corrosion, or looseness, paying particular attention to the PCM/TCM harness connector and the cluster bulkhead connector where this circuit typically routes
  • Check for recent aftermarket modifications, electrical accessories, or prior repairs (such as aftermarket gauges, stereo installs, or trailer wiring) that may have tapped into or damaged the shift lamp control circuit
  • Review technical service bulletins (TSBs) and PCM/TCM software calibration updates for the specific model year, as GM has issued calibration updates addressing erratic upshift lamp operation on certain truck and van platforms

Step 2: Diagnostic Verification

  • Connect a scan tool and review freeze frame data and live data parameters, including commanded upshift lamp state, actual lamp feedback status (if monitored), engine RPM, and vehicle speed at the time the fault was set
  • Command the shift indicator lamp on and off with a bi-directional scan tool, if supported, and observe whether the lamp responds correctly; failure to respond points to a circuit or lamp fault rather than a false code
  • Perform a voltage drop test on the control circuit under commanded load, with the lamp energized; excessive voltage drop (typically more than 0.1ā€“0.2V across any single connector or 0.5V across the full circuit) indicates a corroded connector or damaged wiring
  • Perform a continuity and resistance check of the control circuit wiring between the module connector and the lamp connector with the harness disconnected and key off; resistance should read close to 0 ohms (near-continuity) with no significant resistance indicating a partial break
  • Check for shorts to ground and shorts to voltage on the control circuit with the connector disconnected from both the module and the lamp, using a multimeter referenced to a verified good chassis ground
  • Verify module ground and power supply integrity at the PCM/TCM connector using a digital multimeter against a known good engine or chassis ground, confirming the module reference voltage and ground circuits meet expected values per the wiring diagram before condemning the module

Step 3: Resolution & Reset Process

  • Repair or replace damaged wiring, connectors, or terminals identified during testing using OEM-style weatherproof splices or connector pigtails rather than simple twist-and-tape repairs
  • Replace the shift indicator lamp, bulb, or LED cluster segment assembly if it fails functional (bi-directional command) testing
  • Replace the faulty module (TCM, PCM, or instrument cluster, depending on which component controls the circuit on this specific platform) only after all wiring, connectors, grounds, and power supplies have been verified good
  • Reprogram or update PCM/TCM software using the appropriate factory scan tool/J2534 pass-through device if a calibration update or TSB applies to the VIN
  • Clear all diagnostic trouble codes with a scan tool and perform a key-cycle relearn if required by the module
  • Perform a road test through multiple upshift and downshift conditions to confirm the shift indicator lamp illuminates and extinguishes correctly under all expected shift points
  • Re-scan all modules to confirm no codes return and that live data for the upshift lamp circuit matches commanded and actual states

šŸ“¦ Parts, Labor, and Budget Estimates

Required Components/Software:

  • OEM-level or quality aftermarket scan tool with bi-directional control capability (such as a GM-compatible scan tool or a professional-grade scanner with enhanced GM data)
  • Digital multimeter and factory wiring diagrams specific to the model year, available through GM service information subscription or equivalent repair database
  • Possible parts: shift indicator bulb or cluster LED segment, connector repair kit/pigtail, wiring harness section, TCM/PCM, or instrument cluster (only if verified faulty through testing)
  • Module reprogramming software or dealer calibration via GM Global Diagnostic System (GDS2) or a J2534 pass-through programming interface, if required

Average Component Price:

  • Bulb or indicator cluster segment/assembly: $10ā€“$60
  • Connector or wiring repair kit (pigtail, terminals, splice materials): $20ā€“$120
  • Replacement instrument cluster: $200ā€“$600 (plus programming)
  • Replacement TCM or PCM (if verified faulty): $300ā€“$900 (plus programming)
  • Diagnostic fee: $100ā€“$180 (typically 1ā€“1.5 hours shop diagnostic time)
  • Module programming/calibration fee (dealer or independent shop with J2534 capability): $100ā€“$300

Estimated Labor Hours:

  • Diagnosis and verification (scan tool data review, voltage drop, continuity testing): 1.0ā€“1.5 hours
  • Wiring or connector repair: 0.5ā€“1.5 hours, depending on harness accessibility
  • Instrument cluster replacement (if required): 0.5ā€“1.0 hour
  • TCM/PCM replacement and programming (if required): 1.0ā€“2.5 hours
  • Final verification and road test: 0.3ā€“0.5 hour