The MG ZR, ZS and ZT use three distinct electrical architectures that must be identified before ordering. Pre-August 2003 ZR and ZS cars use a separate Body Control Unit (BCU) and 5AS alarm module mounted behind the glovebox.
From August 2003 onwards (VIN RF 732814 for ZR / Rover 25, VIN RT 637209 for ZS / Rover 45), MG-Rover replaced both modules with the integrated Pektron Security Control Unit (SCU), which combined the BCU, 5AS alarm, MFU timing module, driver's window ECU and flasher relay in a single unit. The MG ZT (and Rover 75) inherited a BMW-derived BCU (part YWC112330) using K-bus communication between the BCU, instrument pack, climate control, immobiliser and fuel-burning heater, a fundamentally different system from the Pektron architecture used in the smaller cars. Identifying the car, year and (on the ZR and ZS) the VIN cutover point is essential when ordering any control-unit related part.
K-Series Engine Sensors and Ignition
The K-series engines in the ZR, ZS and ZT 160 turbo share a common bank of engine sensors which MGOC Spares catalogues directly. The crankshaft position sensor (NSC100760) is the standard service item, a worn or contaminated crank sensor is a typical cause of intermittent non-start and stall faults. The MAP sensor (MHK100820) covers the 1.4 and 1.8 K-series MEMS injection cars, and the coolant temperature sender (YCB100420) is shared with the MGF and MGTF, a useful cross-reference for owners restoring a cold-running gauge fault. HT leads on the 1.8 K-series and KV6 cars are catalogued as the ZS/T 2.0/2.5 set (NGC000020).
The MGB GT V8 spark plug (Champion RN9YC, GSP4462) appears in this catalogue alongside the modern parts, a reminder that the wider MGOC range covers the original RV8 as well as the MG Rover-era cars.
Pektron SCU and Common Relay Failures
The Pektron SCU is the most commonly replaced electrical module on post-2003 ZR and ZS cars. The unit contains five small twin-coil relays on a multi-layer PCB, and the typical failure mode is a fractured coil wire opening one relay coil, a working coil reads 170 to 220 ohms across pins 4 and 8 (or pins 4 and 3) on a multimeter. Symptoms include central locking that locks but does not unlock (or vice versa), the driver's window operating one direction only, horn or fog lights not working, washers not working, and intermittent wipers or super-locking inoperative. Specialist relay-repair services rebuild the SCU economically rather than replacing it, which preserves the original coding and remote fobs.
The SCU is mounted behind the heater control central column on the MGTF and behind the glovebox / aircon evaporator housing on the ZR, ZS and Streetwise, water ingress from a blocked sunroof drain or aircon condensate is a recognised cause of premature failure. A 30-minute battery disconnect will reset minor K-bus glitches but cannot fix a hardware-faulted SCU.
ZT, ZT-T and CDTi Plenum Drains and Water Ingress
The MG ZT and ZT-T inherit a recognised water-ingress fault from the Rover 75 platform: the engine bay plenum chamber drains via four rubber-grommetted holes, and when the grommets are missing or the drains block with vegetation, water collects in the plenum and can flood the passenger-side engine ECU. The grommets are roughly four inches long and direct water to ground past a bulkhead seam, a missing or blocked grommet is a leading cause of catastrophic ECU failure. Owners are advised to inspect the plenum drains as part of an annual service, particularly on cars parked under trees. A separate water-ingress fault affects the boot-mounted sat-nav unit near the driver's side light cluster, where a perished tail-light gasket allows water into the boot floor and saturates the sat-nav module.
Both faults are best addressed before they cause module failure, as replacement modules require T4 diagnostic coding.
Switches, Relays and Heater Blower Resistors
Beyond the major control units, MGOC Spares catalogues the routine switches and relays that owners service most frequently. The heater blower resistor pack (JGH10002) is one of the most commonly failed parts on Rover 25, Rover 45, ZR and ZS cars, a failed pack typically loses the lower fan speeds while the highest speed continues to work, because the highest speed bypasses the resistor entirely. The post-2003 main engine relay (YWB100970) is shared with the MGF and MGTF and is the standard replacement when an engine refuses to crank but the immobiliser disarms correctly. A smaller black relay (YWB000440) is also catalogued for general use.
Switches in stock include the front fog light switch (YUG101680PMP) and the post-2004 facelift driver's window switch (YUF101360PMP).