Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
1999 Mercury Sable V6-3.0L DOHC VIN SFACTORY OEM SERVICE PROCEDURE
TH - Reverse Switch (RS)
TH: REVERSE SWITCH (RS)
TH - DTC P0812 Check RS Switch Function
NOTE: During KOEO Self-Test, gearshift lever in NEUTRAL.
Key on, engine off.
Access RS PID and observe PID cycling ON/OFF with gear shift lever in and out of REVERSE.
Does reading change from ON to OFF?
YES KEY OFF. Fault intermittent. DISCONNECT PCM. INSPECT both ends of the connector for damaged or pushed out pins, corrosion, loose wires. REPAIR as necessary. If OK, REPLACE PCM.
NO KEY OFF. GO to TH2.
TH2 - Check RS Switch Resistance
Locate the reverse switch (RS) near the transmission shift linkage.
Inspect switch and bracket for damage, bent or broken conditions. Repair as required.
Disconnect RS harness connector.
Measure the resistance between the reverse switch terminals with the shift lever in reverse.
Is the resistance less than 5.0 ohms?
YES GO to TH3.
NO REPLACE the RS switch.
TH3 - Check RS Circuit For Open In Harness
Disconnect scan tool.
Disconnect PCM.
Measure resistance of the RS circuit between PCM harness connector pin and RS harness connector.
Where applicable, measure resistance of the SIG RTN circuit between PCM harness connector pin and RS harness connector.
Are both resistances less than 5.0 ohms?
YES GO to TH4.
NO REPAIR open circuit.
TH4 - Check RS Circuit For Short To SIG RTN Or Chassis Ground In Harness
Measure resistance between RS and SIG RTN circuits at the PCM harness connector.
Measure resistance between RS circuit at the PCM harness connector and chassis ground.
Are both resistances greater than 10,000 ohms?
YES REPLACE PCM.
NO REPAIR short circuit.
TH - DTC P0812 Check RS Switch Function
NOTE: During KOEO Self-Test, gearshift lever in NEUTRAL.
Key on, engine off.
Access RS PID and observe PID cycling ON/OFF with gear shift lever in and out of REVERSE.
Does reading change from ON to OFF?
YES KEY OFF. Fault intermittent. DISCONNECT PCM. INSPECT both ends of the connector for damaged or pushed out pins, corrosion, loose wires. REPAIR as necessary. If OK, REPLACE PCM.
NO KEY OFF. GO to TH2.
TH2 - Check RS Switch Resistance
Locate the reverse switch (RS) near the transmission shift linkage.
Inspect switch and bracket for damage, bent or broken conditions. Repair as required.
Disconnect RS harness connector.
Measure the resistance between the reverse switch terminals with the shift lever in reverse.
Is the resistance less than 5.0 ohms?
YES GO to TH3.
NO REPLACE the RS switch.
TH3 - Check RS Circuit For Open In Harness
Disconnect scan tool.
Disconnect PCM.
Measure resistance of the RS circuit between PCM harness connector pin and RS harness connector.
Where applicable, measure resistance of the SIG RTN circuit between PCM harness connector pin and RS harness connector.
Are both resistances less than 5.0 ohms?
YES GO to TH4.
NO REPAIR open circuit.
TH4 - Check RS Circuit For Short To SIG RTN Or Chassis Ground In Harness
Measure resistance between RS and SIG RTN circuits at the PCM harness connector.
Measure resistance between RS circuit at the PCM harness connector and chassis ground.
Are both resistances greater than 10,000 ohms?
YES REPLACE PCM.
NO REPAIR short circuit.
Factory Service Tools
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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