B1557 — Ignition Run/Start Circuit Short To Battery

OBD2 code B1557 flags a problem with the ignition system (ignition run/start circuit short to battery). Short, gentle driving to a shop is usually possible. Common culprits: moisture or oil in the spark plug wells, faulty ignition control circuit in the ECM, worn spark plugs or wrong plug gap.

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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

Read the code and freeze frame Connect an OBD2 scanner, confirm B1557 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
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.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the ignition system for damage, corrosion, chafing or poor contact.
Subsystem check Swap the coil to another cylinder and see if the fault code follows it.
Subsystem check Inspect spark plugs for wear, oil or carbon tracks and check plug gaps.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm B1557 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for B1557 (parts and labor, USD, independent shop rates): Replace an ignition coil: $150–$380; Repair coil wiring or connector: $80–$250; Replace spark plugs: $90–$250. Overall, fixing B1557 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

Will B1557 clear itself?

Sometimes B1557 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

What does OBD2 code B1557 mean?

What is the most common cause of B1557?

The most common causes are: moisture or oil in the spark plug wells, faulty ignition control circuit in the ECM, worn spark plugs or wrong plug gap.

How much does it cost to fix B1557?

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

Is it safe to drive with B1557?

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.

When your vehicle's computer detects a problem with the ignition run/start circuit, it logs diagnostic trouble code B1557. This code specifically indicates that the circuit is experiencing a short to battery voltage — meaning electricity is flowing where it shouldn't, or the system is seeing a voltage level that exceeds normal operating parameters. The engine control module continuously monitors this circuit while the engine runs, comparing real-time readings against factory specifications. When those readings stray outside acceptable ranges for enough consecutive drive cycles, the module stores B1557 and illuminates the Check Engine Light.

Understanding why this matters requires a quick look at how the ignition system operates. The ignition run/start circuit carries electrical signals that tell the engine when to fire the spark plugs. When that circuit malfunctions due to a short, the timing of spark events becomes erratic or stops entirely. The control module saves freeze frame data alongside the code — a snapshot of engine speed, load, temperature, and vehicle speed at the exact moment the fault occurred. That data is invaluable during diagnosis because it tells you what conditions triggered the problem.

Once B1557 sets, several things happen. The module switches to failsafe mode, which means it may rely on default ignition timing instead of the optimized values it normally uses. This explains why the engine runs poorly even though the underlying problem might seem minor. The vehicle remains drivable in most cases, but performance suffers and further damage becomes a real risk if the issue persists.

Common Symptoms of B1557

Check Engine Light remains illuminated after startup
Engine misfires during acceleration or at idle
Difficulty starting the engine, particularly in damp or rainy conditions
Noticeable loss of engine power and sluggish response
Rough or uneven idle that fluctuates without driver input
Fuel economy drops noticeably compared to normal driving
In severe cases, the engine stalls unexpectedly

What Causes B1557?

Moisture or oil accumulation in the spark plug tube wells — this creates a conductive path that causes shorts
Spark plugs that are worn out, fouled with carbon deposits, or gapped incorrectly
Failed ignition coil or coil pack unable to deliver proper voltage
Damaged wiring harness or loose connectors in the ignition control circuit
Corrosion or oxidation at electrical connection points
Internal fault within the engine control module itself (relatively uncommon)
Aftermarket accessories incorrectly wired into the ignition circuit

How to Diagnose B1557

Connect an OBD2 scanner and retrieve all stored codes. Note B1557 specifically and record the freeze frame data — this shows exactly what the engine was doing when the fault was detected and guides your testing.
Look for any related diagnostic trouble codes. Codes in the B0xxx or P0xxx families often point to the underlying cause. Check manufacturer TSB databases for known issues affecting your specific make and model — many codes have documented factory fixes or software updates.
Perform a thorough visual inspection of all wiring and connectors associated with the ignition system. Look for damaged insulation, broken wires, corrosion buildup, and any signs that connectors are not fully seated.
Test the ignition coil output with a multimeter or oscilloscope. If equipment is unavailable, swap the suspected coil with one from a cylinder that is running properly and observe whether the fault code moves with the part.
Remove and inspect the spark plugs. Check for black carbon deposits, oil fouling, worn electrodes, or gaps that deviate from manufacturer specifications.
Clear B1557 from the system, then drive the vehicle through a complete drive cycle — particularly under the conditions recorded in the freeze frame data. If the code returns, the root cause has not been addressed.

Repair Options for B1557

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

Always address the least expensive likely causes before replacing major components. In many cases, cleaning corroded connectors or installing new spark plugs eliminates the code without touching the coil or control module.

How Much Does It Cost to Fix B1557?

Total repair costs for B1557 vary based on which component is actually at fault. At independent shop rates in the United States, expect to pay between $80 and $380 for parts and labor combined. Replacing a single ignition coil runs $150–$380 depending on vehicle make. Wiring harness repairs typically fall between $80 and $250. Spark plug replacement, including labor, generally costs $90–$250 for a full set. Getting an accurate diagnosis before purchasing parts is the single most effective way to keep costs manageable — the most expensive repair scenario is rarely the first one you should check.

Can You Drive with B1557?

Use caution if the Check Engine Light is solid. Avoid hard acceleration and heavy loads because misfiring cylinders send unburned fuel into the exhaust system, where it can ignite inside the catalytic converter and cause catastrophic damage. If the light flashes instead of remaining steady, stop driving immediately — a flashing Check Engine Light signals an active misfire that demands prompt repair. Regardless of light behavior, schedule diagnosis as soon as reasonably possible to prevent secondary damage to emissions components.