P0720 on Mercedes-Benz: How to Diagnose & Fix

The 722.9 7G-Tronic transmission conductor plate (internal electrical board with speed sensors) fails, causing limp mode, harsh shifting, and loss of gears. The electrical connector corrodes or the sensors crack from heat cycling.

The 722.9 7G-Tronic automatic transmission develops valve body issues causing harsh or delayed shifts, particularly the 2-3 and 4-5 upshifts. The conductor plate (electrical connector inside the valve body) is also failure-prone.

The 722.6 five-speed automatic (fitted to W202 facelift cars from ~1996 onward and all 2001-2002 W203s) uses an electronic conductor plate mounted on top of the valve body that integrates the speed sensors and routes signals to the solenoids. The plate fails for two reasons: (1) the internal RPM sensors get coated by ferrous wear particles when the fluid is never changed, producing implausible-slip codes, and (2) the soldered traces and embedded capacitor crack from heat cycling. Failure puts the transmission into limp-home mode — stuck in 2nd gear, harsh 1-2 and 2-3 shifts, or no upshift at all. Often accompanied by codes P0717, P0720, P0731-P0736 and a transmission warning light. This is the single most common 722.6 failure across every Mercedes chassis that uses it.

The 722.9 7G-Tronic automatic transmission used in later R171 models is known for conductor plate and internal speed sensor failures. When the plate or sensors fail, the transmission may enter limp mode, shift harshly, or get stuck in one gear due to loss of accurate input/output speed data.

The electronic conductor plate integrated into the 722.9 7-speed automatic's valve body (which houses the internal TCM, speed sensors and shift solenoids) is a widely reported failure on R171 SLKs equipped with the 7G-Tronic. As the plate's internal speed sensors and soldered joints degrade with heat cycling, the transmission drops into limp mode, typically locking into a single forward gear (often 2nd). The condition is temperature-sensitive and commonly appears from roughly 50,000-90,000 miles.

Sprinters with the 5-speed 722.6 (NAG1) automatic use an internal conductor plate that carries the shift-solenoid wiring and the input/output speed sensors. The conductor plate is a known weak point: when it fails the transmission control unit gets bad speed signals, lights the warning lamp and drops the van into limp-home mode, typically stuck in a single gear (often 2nd). Symptoms include harsh or delayed shifts and an inability to shift out of second.

The 722.9 7G-Tronic automatic transmission fitted to ML350, ML500/550 and other gasoline M-Class models uses an internal conductor plate that doubles as the transmission control module and houses the input/turbine speed sensors. Heat, ATF breakdown and vibration cause the molded-in speed sensors and the plate's electronics to degrade internally. The transmission then loses the ability to monitor input-shaft speed and torque-converter slip, producing harsh, delayed or 'thumping' shifts that start out intermittent and eventually become permanent. In the worst case the transmission locks into a single gear (typically 2nd) and enters limp mode. This is one of the most widely reported 7G-Tronic faults across the W164/W166 M-Class. Repair is plug-and-play once codes are cleared with a STAR/equivalent scanner.