Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Verifying Repairs
For catalytic converter, HO2S, EGR, fuel trim and EVAP system related DTCs, it will be necessary to drive vehicle according to drive cycle that allows PCM to monitor the fault area. Clear DTCs and drive vehicle according to appropriate monitor. See appropriate monitor under OBD-II DRIVE CYCLES .
If drive cycle used to verify repair runs catalytic converter monitor, HO2S monitor, EGR monitor, fuel trim monitor, or EVAP system leak monitor, repair may be verified in one drive cycle by checking readiness test status. Readiness test status is an indicator showing whether monitor has run or not run. If PCM ran monitor and found no faults, readiness test status will display COMPLETE or READY. Repair is verified.
If PCM found fault during drive cycle, or monitor did not meet criteria to run, readiness test status will display INCOMPLETE or NOT READY. Perform drive cycle again. The second time drive cycle is run, readiness test status will display COMPLETE or READY whether or not a fault was found. It will be necessary to check for DTCs. If readiness test status does not display COMPLETE or READY after 2 drive cycles, one or both drive cycles failed to meet criteria necessary for PCM to run monitor.
For systems and components monitored by other monitor, record freeze frame data. Clear DTCs. Drive vehicle in manner approximating conditions recorded in freeze frame in order to verify repair.
Other monitor does not have readiness test status because other monitor runs continuously. Manufacturer's MUT-II Scan Tool (MB991502) provides Mode 2 (increased sensitivity) and Mode 5 (provisional DTCs) functions which allow repairs to be verified after one drive cycle. Generic scan tools may or may not have these functions. If scan tool does not have these functions, drive vehicle under conditions approximating freeze frame data twice, then check for DTCs.
Scan tool Mode 2 (increased sensitivity) allows single drive cycle testing of emissions related DTCs. Mode 2 commands PCM to illuminate SERVICE ENGINE SOON/MIL and set DTC on first occurrence of fault when code set criteria are being met. If SERVICE ENGINE SOON/MIL does not illuminate during drive cycle, repair is verified. Return to Mode 1 (normal) by turning ignition switch to OFF position or selecting Mode 1. When scan tool returns to Mode 1, freeze frame data, readiness test status and DTCs are erased.
Scan tool Mode 5 (provisional DTCs) can be used to verify repairs after only one drive cycle by reporting fault as provisional (pending) DTC. Mode 5 is limited to reporting faults related to misfire, fuel system and faults detected by other monitor.
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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.
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