B1303 — Accessory Delay Relay Coil Circuit Open

body electrical

Is it safe to drive?

Drive with caution Drive with caution and avoid load. Ignition faults cause misfires that quickly overheat and destroy the catalytic converter; if the Check Engine Light flashes, stop and fix it first.

How to diagnose with an OBD scanner

  1. Read the code and freeze frame Connect an OBD2 scanner, confirm B1303 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
  2. Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many B0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
  3. Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the ignition system for damage, corrosion, chafing or poor contact.
  4. Subsystem check Do a continuity test end-to-end on the accessory delay relay coil circuit and wiggle the harness while measuring — intermittent opens show up as fluctuating readings.
  5. Subsystem check Swap the coil to another cylinder and see if the fault code follows it.
  6. Subsystem check Inspect spark plugs for wear, oil or carbon tracks and check plug gaps.
  7. Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm B1303 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for B1303 (parts and labor, USD, independent shop rates): Replace spark plugs: $90–$250; Replace an ignition coil: $150–$380; Repair coil wiring or connector: $80–$250. Overall, fixing B1303 usually costs between $80 and $380. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

How do I diagnose B1303 myself?

Start with a scan: confirm B1303, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the accessory delay relay coil, and verify the suspect component against live data before buying parts.

What are the symptoms of B1303?

Drivers most often notice: loss of power and jerking, increased fuel consumption, Check Engine Light on, hard starting, especially in damp weather.

What is the most common cause of B1303?

The most common causes are: moisture or oil in the spark plug wells, faulty ignition control circuit in the ECM, damaged coil wiring or connector.

Is it safe to drive with B1303?

Drive with caution and avoid load. Ignition faults cause misfires that quickly overheat and destroy the catalytic converter; if the Check Engine Light flashes, stop and fix it first.

How much does it cost to fix B1303?

Replace spark plugs — $90–$250; Replace an ignition coil — $150–$380; Repair coil wiring or connector — $80–$250. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

The powertrain control module logs B1303 when it detects that the accessory delay relay coil circuit has lost electrical continuity. This particular circuit is part of the vehicle's body electrical architecture and manages power delivery to various accessories. When the module's self-diagnostics run through their verification cycle—typically requiring confirmation across multiple drive cycles before setting an official fault code—they compare live sensor readings against factory-calibrated thresholds. If the readings fall outside acceptable parameters and the module cannot establish a complete circuit through the accessory delay relay coil, B1303 gets stored in memory. Alongside the code, the module captures a freeze frame snapshot recording engine speed, load percentage, coolant temperature, and vehicle speed at the precise moment the fault first occurred. That data proves invaluable during diagnosis because it tells you exactly what conditions triggered the problem.

"Circuit open" is diagnostic shorthand for a broken electrical path. In practical terms, current cannot flow where the module expects it to. This interruption can stem from several sources: physical damage to the wiring harness, a connector that has come loose or developed corrosion at its pins, a fuse that has opened under excess current draw, or the relay coil itself having an internal failure that prevents it from functioning. The key point is that B1303 indicates a complete break in the circuit rather than a short to ground or battery voltage. Until that path is restored, the affected system cannot operate as designed.

Diagnosing this code effectively requires understanding the module's logic. The control unit does not guess—it has specific expectations about voltage and resistance at each circuit point. When those expectations go unmet, it flags the fault. By reviewing the freeze frame data first, you can often recreate the exact conditions (load, temperature, speed) under which the problem appeared, making intermittent faults easier to trigger and confirm during testing.

Symptoms of B1303

  • Noticeable loss of engine power paired with jerky or stuttering acceleration
  • Fuel economy worse than normal for the same driving patterns
  • Check Engine Light illuminated on the instrument cluster
  • Difficulty starting the engine, particularly in humid or wet conditions
  • Misfires occurring either at idle or when the engine is under load

Common Causes

  • Broken or severed wire within the accessory delay relay coil harness
  • Connector unplugged, pins backed out, or significant corrosion at the connection points
  • Internally open accessory delay relay coil that no longer passes current
  • Blown fuse protecting the accessory delay relay coil circuit
  • Worn or incorrectly gapped spark plugs contributing to secondary ignition problems
  • Damaged wiring or connectors feeding the ignition coil or coil pack
  • Moisture or oil contamination in the spark plug tube wells
  • Failed ignition coil or coil pack with internal open circuit
  • Faulty ignition control signal from the engine control module

How to Diagnose B1303

  1. Connect an OBD2 scanner to the vehicle's diagnostic port, retrieve B1303 from stored codes, and record the freeze frame data—this shows what the engine was doing when the fault first appeared.

  2. Check for any related diagnostic trouble codes. Codes in the B0xxx series often share common root causes, and manufacturers may have published technical service bulletins with known fixes or software updates for your specific model year.

  3. Perform a detailed visual inspection of the wiring harness and all connectors associated with the accessory delay relay coil circuit. Look for chafed insulation, broken strands, corrosion buildup, and any connectors that are not fully seated.

  4. Using a digital multimeter, test for continuity end-to-end along the accessory delay relay coil circuit. Gently wiggle the harness while monitoring readings—an intermittent open will cause the multimeter to fluctuate or show OL.

  5. If the vehicle exhibits misfire symptoms alongside B1303, swap the ignition coil to a different cylinder and track whether the fault code moves with the coil.

  6. Remove and inspect the spark plugs for wear, carbon fouling, oil deposits, or damaged electrodes, and verify that each plug's gap falls within manufacturer specification.

  7. Clear the code from the module's memory, complete a full drive cycle that includes the conditions recorded in the freeze frame, and verify that B1303 does not reappear.

Repair Options

  • Spark plug replacement: $90–$250
  • Ignition coil replacement: $150–$380
  • Coil wiring or connector repair: $80–$250

Begin with the least expensive and most frequently encountered causes before committing to costly component replacements. Wiring damage and poor connector contact account for a substantial percentage of circuit-open complaints.

Total repair costs for B1303 typically fall between $80 and $380 depending on which component is at fault. An accurate diagnosis