B1847 — Ignition Tamper Circuit Short To Battery

OBD2 code B1847 flags a problem with the ignition system (ignition tamper circuit short to battery). Short, gentle driving to a shop is usually possible. Common culprits: faulty ignition control circuit in the ECM, moisture or oil in the spark plug wells, worn spark plugs or wrong plug gap.

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

Read the code and freeze frame Connect an OBD2 scanner, confirm B1847 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 B1847 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for B1847 (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 B1847 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 B1847 clear itself?

Sometimes B1847 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 is the most common cause of B1847?

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

What are the symptoms of B1847?

Drivers most often notice: hard starting, especially in damp weather, misfires under load or at idle, increased fuel consumption, loss of power and jerking.

Is it safe to drive with B1847?

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 B1847?

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.

Your scanner just displayed code B1847, and now you need to understand what that means for your vehicle. B1847 indicates that the powertrain control module has detected the ignition system operating outside its intended parameters. The technical description reads "Ignition Tamper Circuit Short To Battery," which falls under body electrical fault categories. Put simply, the circuit that monitors ignition system integrity is seeing a voltage signal it should not see — a direct battery voltage present where only switching signals should exist.

The control unit does not flag this fault on a single anomaly. Automotive ECUs are designed to confirm abnormal conditions across multiple ignition cycles before illuminating the Check Engine Light and storing the code. When B1847 triggers, the module captures freeze frame data — engine load, RPM, coolant temperature, and vehicle speed recorded at the exact moment the fault occurred. That snapshot becomes your roadmap for diagnosis, since replicating those specific conditions makes the problem far easier to reproduce and isolate.

The ignition tamper circuit exists as a protective monitor. It watches for unauthorized modifications to the ignition system as well as internal faults within the coil drivers or related wiring. When battery voltage appears on that circuit unexpectedly, the ECU interprets it as a tampering condition or a short circuit, and B1847 gets set accordingly.

Common Symptoms of B1847

When B1847 is active, you may notice one or more of the following:

Engine cranks slowly or fails to start, particularly in wet or humid conditions
Misfires occurring either at idle or under acceleration
Noticeable increase in fuel consumption compared to normal driving patterns
Reduced engine power with hesitation or jerking during acceleration
Check Engine Light remains illuminated on the dashboard

What Causes B1847?

The underlying issue typically originates in one of several places within the ignition circuit:

Damaged or corroded wiring feeding the ignition coil, often from age, heat exposure, or road debris
Failed ignition coil or coil pack that internally shorts to battery voltage
Faulty ignition control circuitry within the engine control module itself
Worn or contaminated spark plugs creating erratic secondary voltage behavior
Improper spark plug gap altering the load the coil must fire against

In most cases, the problem traces back to deteriorating coil wiring or a failing coil unit, with ECM faults representing a less frequent but still possible root cause.

How to Diagnose B1847

Follow these steps to isolate the root cause before replacing any components:

Connect an OBD2 scanner and confirm B1847 is stored in the ECU. Save the associated freeze frame data, which documents engine conditions at the moment the fault was detected — replicate those conditions during your road test.
Check for any additional diagnostic trouble codes. Related codes often point directly to the failing component and can significantly narrow your focus.
Inspect the wiring harness and connectors at the ignition coil for physical damage, corrosion, melted insulation, or loose terminals. Pay special attention to areas near heat sources or moving engine components.
Test the ignition coil resistance specifications against manufacturer values, or swap the suspected coil with a known-good unit from another cylinder to see if B1847 follows the coil.
Remove and examine the spark plugs for excessive wear, oil fouling, carbon tracking, or incorrect gap settings.
Clear the code after completing your inspections, then drive the vehicle through a complete drive cycle including the conditions captured in the freeze frame. If B1847 returns, the fault remains active and further diagnosis is required.

Repair Options and Costs

Once the failing component is identified, repair costs for B1847 typically break down as follows:

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

All prices reflect typical parts and labor at independent repair shops in the United States. The exact total depends entirely on which part failed and your vehicle's accessibility requirements.

Addressing B1847 generally costs between $80 and $380 depending on the root cause. The most expensive repair scenario — ECM replacement — is rarely necessary and should only be considered after all other components have been ruled out. Starting with visual inspection and testing the coil and wiring saves money and prevents unnecessary part swapping.

Driving Safety Verdict

Caution is the appropriate mindset when B1847 is present. While a steady Check Engine Light does not demand immediate roadside repair, driving aggressively with an active ignition fault risks serious damage. Misfires from coil or plug failures push unburned fuel into the exhaust system, and that raw fuel overheats the catalytic converter rapidly — an extremely expensive repair. If the Check Engine Light begins flashing, stop driving immediately and have the vehicle towed to a repair facility. Even with a steady light, schedule diagnosis at your earliest convenience to prevent escalating damage.