Reverse Input Circuit
P0812 indicates the PCM has detected a problem with the reverse input circuit, which monitors when the transmission is in reverse gear. This code sets when the reverse position sensor signal is abnormal, shorted, or not communicating properly with the engine control module.
What You Might Notice
- Reverse lamps do not illuminate when shifting to reverse
- Reverse lamps stay on continuously even when not in reverse
- Transmission will not shift into reverse or reverse engagement is delayed
- Check engine light illuminated on dashboard
- No backup camera display on vehicles equipped with camera systems
Most Common Causes
- 1
Defective reverse position sensor
The reverse position sensor may have failed or become stuck, preventing proper signal transmission to the PCM. This is the most common cause of P0812.
- 2
Faulty neutral safety switch
A bad neutral safety switch can cause incorrect reverse signal detection and prevent proper reverse lamp control. This switch is critical for reverse circuit operation.
- 3
Shorted or damaged wiring harness
Wiring in the reverse input circuit may be corroded, pinched, or shorted to ground, disrupting the signal between the sensor and PCM. Check connectors and wires for damage.
- 4
PCM programming error or module failure
The powertrain control module may have corrupted programming or internal failure preventing proper reverse circuit signal interpretation. This requires module reprogramming or replacement.
How to Diagnose It
- 1
Check Defective reverse position sensor — The reverse position sensor may have failed or become stuck, preventing proper signal transmission to the PCM. This is the most common cause of P0812.
- 2
Check Faulty neutral safety switch — A bad neutral safety switch can cause incorrect reverse signal detection and prevent proper reverse lamp control. This switch is critical for reverse circuit operation.
- 3
Check Shorted or damaged wiring harness — Wiring in the reverse input circuit may be corroded, pinched, or shorted to ground, disrupting the signal between the sensor and PCM. Check connectors and wires for damage.
- 4
Check PCM programming error or module failure — The powertrain control module may have corrupted programming or internal failure preventing proper reverse circuit signal interpretation. This requires module reprogramming or replacement.
How to Fix It
- 1
Test and replace reverse position sensor
Use a digital volt/ohmmeter to test the reverse position sensor for proper voltage and resistance values when the transmission is shifted to reverse. Replace the sensor if it does not meet manufacturer specifications.
- 2
Inspect and repair wiring harness
Inspect all wiring connections in the reverse input circuit for corrosion, damage, or loose connections. Repair shorted wires or replace corroded connectors as needed to restore signal integrity.
- 3
Test and replace neutral safety switch
Test the neutral safety switch to verify it signals reverse gear properly. If the switch does not activate reverse circuits when shifted to reverse, replace it with an OEM or quality replacement.
- 4
Reprogram or replace PCM
If all sensors and wiring test normal, connect to vehicle-specific diagnostic software to reprogram the PCM or reflash the module. If reprogramming fails, the PCM module may require replacement.
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