OBD Code B1691: Autolamp Delay Circuit Short To Battery

Q: What does OBD code B1691 mean?

Autolamp Delay Circuit Short To Battery. The autolamp delay circuit is detecting a short-to-battery condition, meaning power is flowing where it shouldn't be. Think of it like a water pipe that's cracked and leaking pressurized water into an area where it shouldn't go.

Q: How do I fix OBD fault code B1691?

Inspect and reseat all connectors in the autolamp circuit for corrosion or loose pins causing intermittent shorts.. Replace the autolamp delay relay if testing confirms it has shorted internally to battery voltage.. Check for pinched, abraded, or damaged wiring harness near engine bay heat sources that may have created an unintended short path.

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What This Actually Means

In plain language — no jargon

The autolamp delay circuit is detecting a short-to-battery condition, meaning power is flowing where it shouldn't be. Think of it like a water pipe that's cracked and leaking pressurized water into an area where it shouldn't go.

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Symptoms You May Notice

3 known symptoms for this code

Headlights or daytime running lights stay on when they shouldn't

Dashboard lighting control malfunction or erratic behavior

Battery drains faster than normal due to continuous power draw

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Embedded Systems Insight

What the ECU/ECM is actually computing

The ECU monitors the autolamp delay circuit voltage to ensure it stays within expected operating range. When the circuit shorts to battery voltage (typically 12-14V), the ECU detects abnormally high voltage where a controlled, lower voltage signal should exist. The module compares this against safe thresholds to trigger the fault.

Voltage & Parameter Thresholds

Parameter	Normal Range	Fault Condition
Autolamp Delay Circuit Voltage	0.5-4.5V (controlled signal)	>11V (shorted to battery)
Circuit Resistance	500-5000 ohms	<100 ohms (short condition)

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Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex

1

Wiring harness connectors

Inspect and reseat all connectors in the autolamp circuit for corrosion or loose pins causing intermittent shorts.

2

Autolamp relay

Replace the autolamp delay relay if testing confirms it has shorted internally to battery voltage.

3

Lighting control module wiring

Check for pinched, abraded, or damaged wiring harness near engine bay heat sources that may have created an unintended short path.