NOTE: If any HO2S DTC has been set, this test cannot be used.

Connect scan tool. On California models, monitor left WU-TWC using FHO2SL PID for upstream reading and RHO2SL for downstream reading. Monitor right WU-TWC using FHO2SR PID for upstream reading and RHO2SR for downstream reading. On Federal models, monitor TWC using FHO2S PID for upstream reading and RHO2S for downstream reading. On all models, press START to begin monitoring appropriate PIDS.
Drive vehicle at 40-60 MPH for 10 minutes to warm TWC to operating temperature. Stop vehicle and allow to idle. Press TRIGGER to begin recording PIDs. Wait one minute, then press TRIGGER to end recording PIDs.
Select appropriate PIDs and read graph in VIEW RECORDING. Count the number of times (inversions) the upstream reading crosses the .5 volt line. Count the number of times (inversions) the downstream reading crosses the .5 volt line. Do not count the number of peaks. See Fig 1 .
Compare the inversions of the upstream reading to the downstream reading. If the upstream reading has at least twice as many inversions as the downstream reading or downstream reading has no inversions, TWC is okay.

Fig 1: Identifying Upstream & Downstream HO2S Inversions

Courtesy of MAZDA MOTORS CORP.

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.

Safety Warning

Three-Way CATAYLTIC Converter (MPV)