P0812
Reverse Input Circuit
Powertrain Transmission Control Reverse input circuit 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

Your car's reverse gear signal isn't reaching the engine computer properly, like a broken button that won't send a signal when pressed. The ECU can't confirm the transmission is in reverse, which affects shift logic and safety features.

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Symptoms You May Notice
3 known symptoms for this code
Reverse lights don't illuminate
Transmission won't shift into reverse or shifts erratically
Check Engine Light illuminated
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECU monitors the reverse input circuit voltage signal from the transmission range switch or gear position sensor. It expects a specific voltage transition when reverse is selected. If the signal is missing, shorted, or open, the ECU cannot confirm reverse engagement and sets a fault code.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
Reverse Input Signal Voltage 5V when reverse selected; 0V in other gears No voltage change or incorrect voltage detected
Circuit Continuity Complete circuit with proper resistance Open circuit or short to ground detected
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Transmission range switch connector
Inspect and reseat the reverse switch connector; clean corrosion from pins with contact cleaner.
2
Reverse light switch wiring harness
Check for damaged, pinched, or corroded wires between transmission and ECU; repair or replace as needed.
3
Transmission range switch
Replace the switch if continuity test fails or voltage signal is absent when reverse is selected.