What does it mean?
P1421 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Secondary Air Injector Valve Circuit Short to Ground .
Symptoms
- MIL on and the secondary air monitor does not complete
- Valve stays energised, so air may be injected at the wrong time or continuously
- Lean readings from the upstream oxygen sensors at idle and possible fuel trim codes
- Rough idle or hunting engine speed when warm
- Repeated fuse failures in the air injection circuit
Likely causes
- Control wire rubbed through to ground against a metal edge or exhaust shield
- Water intrusion into the valve connector, common because the valve often sits low in the engine bay
- Internally shorted solenoid winding touching the valve body
- Crushed wiring at a harness clip or where it passes through a bulkhead grommet
- Damaged insulation from previous repair work or rodent damage
- Corrosion inside the connector creating a leakage path to the valve housing
- PCM low side driver shorted internally, only after wiring is proven
How to fix it
- Unplug the valve and measure resistance between the control terminal and ground; near zero means a hard short
- Repeat the measurement with the PCM connector removed to isolate the harness from the module
- Inspect the valve connector for water, corrosion and green terminals, and dry it before retesting
- Measure the solenoid winding to the valve body; any continuity means the valve is internally shorted
- Trace the harness route and look for chafed or crushed insulation, especially near exhaust heat shields
- Repair the shorted wire with proper insulation and secure routing away from hot and sharp surfaces
- Replace the valve if it is internally shorted, then command the valve with a scan tool and confirm correct operation