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Q: I have a 2007 Toyota RAV4 I bought from a large car dealer. After two months the "Check engine" light came on. The Toyota dealer said it was a defective oxygen sensor, which I had replaced. The next day the same light came on again. What control does a dealer have over the check-engine light? Can they set the codes to defer for a period of time? It seems curious that the light came on two months after driving off the lot.

A: Would it have been more "curious" had the check-engine light come on the day after you purchased the vehicle? Without knowing the DTC fault code that has triggered the light this second time, there's no way to know whether the recurrence is related in any way to the original oxygen sensor fault.

Assuming the original DTC pointed to a failed oxygen sensor, the dealer would have replaced it and cleared the fault code from the computer's memory. That means the light coming on a day later was generated by a completely new fault, or the same fault recurring. It's important to remember that there are several fault codes related to oxygen sensor(s) and their performance that do not indicate sensor failure. Replacing an O2 sensor when the fault is a harness/connector/heater or rich/lean air-fuel mixture won't prevent the code from recurring.

To put your suspicions and paranoia to rest, I don't believe there's any way to manipulate an engine management system to "defer" fault codes in order to conceal a problem. And of course, to do so would — like odometer rollback — violate federal law.

Q: I drive a 2003 Chevy Impala with 106,000 miles. Several months ago the "Service traction system" message came on. I took it to the dealer and their diagnostic check found code C1218 — "Pump motor circuit open." Their test found that the ABS pump motor has had an internal failure and needs to be replaced, at a cost of over $2,000. Would it be helpful to flush the hydraulic lines to clean out any debris that might have caused this failure? Would not replacing the pump have any effect on the ABS braking system?

A: According to my ALLDATA automotive database, the C1218 code indicates a voltage issue with the pump motor in the BPMV (brake pressure modulator valve), part of the EBCM (electronic brake control module). A special test lamp can test the EBCM's ability to control the BPMV. If the lamp lights up, the pump motor circuit within the EBCM is good.

Until the cause for the ABS and TCS warning lights and "Service traction system" message is identified and corrected, the ABS and TCS are disabled.

Bleeding/flushing the hydraulic system to remove moisture and debris every two years or so is routine maintenance in my book.

Q: I have a 2002 Chevy 2500HD Silverado with 156,000 miles. About six weeks ago the ABS started activating when stopping from a very low speed with little pressure on the brake pedal. It sounds like the left front wheel. No dash warning lights are on before, during or after this happens. It's like the wheel speed sensor thinks one of the front wheels is locking.

A: That's exactly what's happening! Because of an increased air gap between the wheel speed sensor and reluctor ring on the hub due to a buildup of rust and debris, the sensor can't accurately read the rotational speed of the wheel. And since no rotational reading is interpreted as lockup by the system, the ABS engages to bleed off hydraulic pressure to that brake in an attempt to restore rotation.