P0812
Reverse Input Circuit
General electrical fault on the reverse-engagement input — the signal that tells the TCM (and other modules — backup lights, parking sensors) that the gear lever is in Reverse. With the input invalid backup lights and rear sensors do not activate.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: medium
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
Common causes
Ordered by how often each turns out to be the cause.
- Open or shorted reverse input wireCommon
Parts involved
- Reverse Input Switch
- Wiring Harness
P0812 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.