DTC P0135: Front Heated Oxygen Sensor Heater Circuit Malfunction: Condition

WARNING: This page is about a different car, the 2002 Mazda 626. However, it is still accessible from the selected car via links, so may be relevant.

DTC is set when Powertrain Control Module (PCM) detects input voltage from front heated oxygen sensor (HO2S) heater is less than .14 amp or more than 3 amp after front HO2S heater has been on for 254 seconds. This is an intermittent monitor. MIL is illuminated when PCM detects malfunction during 2 consecutive drive cycles. Possible causes are:

Front HO2S malfunction.
Open in circuit between main relay and front HO2S.
Short or open in circuit between PCM and front HO2S.
PCM malfunction.