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P0135 O2 Sensor Fault on 1998 Mercury Tracer 2.0L LS - Check Engine Light, Idle Roughness and Stalling

Model: Mercury Tracer Fault Code: P0135 Posted: 2012-08-25 15:21

I'm experiencing persistent engine issues on my 1998 Mercury Tracer 2.0L LS with 124,000 miles. Current Problems: 1. Check Engine Light is continuously illuminated — diagnostic scan shows DTC P0135 (Oxygen Sensor Heater Circuit Malfunction). - Replaced the upstream O2 sensor with a Ford OEM unit; issue persists. 2. Idle is rough and occasionally stalls, with noticeable surges during operation. - Fuel pump has been tested and functions normally. - New fuel filters have been installed (two sets). - Injector performance tests show no faults. - No vacuum leaks detected. Additional Question: Does a P013-5 O2 sensor fault directly contribute to idle instability or stalling? I've reviewed related discussions but would appreciate expert insight. Nicolai

Related fault codes
P0135
Comments (4)
Anonymous 2012-09-07 15:15

Following up on the previous post: I rechecked all wiring for shorts and discovered water damage in the harness near the Powertrain Control Module (PCM). The PCM was replaced, and the vehicle runs more smoothly for about two days before P0135 returns. I consulted a Ford dealership — they stated that O2 sensor failure is rare and advised that wiring replacement may be necessary, though parts are not available locally. Any recommendations or troubleshooting steps? Nicolai

Anonymous 2012-09-07 16:05

Before installing the new Ford OEM O2 sensor, did you verify the heater circuit was receiving 12 volts? Also, please confirm the fuel pressure reading in psi — this is critical for diagnosing fuel delivery issues. Thanks for the details!

Anonymous 2012-09-08 13:24

Hello kev2. All fuses have been checked and are functioning properly. I don't recall the exact fuel pressure value, but it was within normal range. Regarding the P0135 code: FUEL SYSTEM 1 = CL (Circuit Low), FUEL SYSTEM 2 = NA, CALC LOAD (%) = 33.7, ECT(F) = 192°F, STFT BI(%) = -4.7. I have not yet tested the heater circuit for 12V power. Nicolai

Anonymous 2012-10-03 17:18

To wrap up: First, is this vehicle actually a female? (Just kidding — it's clearly a car!) After replacing the PCM, the heater core unexpectedly failed. I bypassed the core temporarily and noticed that the check engine light finally turned off. However, the cooling fan did not activate initially but started working later. Could water damage in the wiring harness have caused this failure? With winter approaching, should I invest in a new heater core or consider purchasing a used vehicle instead? Thanks for your input. N.