5.10 No-Code Driveability Diagnosis & Interrelated System Faults

Diagnosing Without a Code

Task E.4 is written because the most difficult real-world complaints have no code. A DTC sets only when a monitored parameter exceeds a calibrated threshold while the monitor's enable criteria are satisfied. Three situations produce a genuine fault with no code:

1. The fault is inside the plausibility window. A coolant temperature sensor reading 60 °C when the engine is actually at 90 °C is wrong, but 60 °C is a plausible value, so no rationality test fails. The PCM enriches fuel and retards timing for a cold engine that is not cold — producing poor economy, hesitation, and a rich exhaust with no code.
2. The monitor never ran. Enable criteria were never met, or the vehicle is driven in a pattern that never satisfies them.
3. The fault is not monitored. Base engine mechanical condition, exhaust restriction, and induction restriction have no dedicated OBD-II monitor.

The parameter-based method

Without a code to anchor the diagnosis, the scan tool data list becomes the primary instrument.

Step 1 — Establish key-on, engine-off plausibility. With the engine cold and off overnight, all temperature sensors must read within a few degrees of each other and of ambient. ECT, IAT, transmission fluid temperature, and fuel temperature disagreeing at cold soak is the fastest way to find a drifted sensor no code will catch.

Step 2 — Sanity-check calculated values.

Step 3 — Read pending codes and Mode $06. A pending code documents a fault that failed once but has not yet met the two-trip requirement. Mode $06 goes further: it returns each monitor's raw test value with its minimum and maximum limits. A catalyst test result at 0.88 against a 0.90 fail limit identifies a converter that will set P0420 within weeks — invisible on a code scan.

Step 4 — Test what is not monitored. Verify mechanical condition (running compression, relative compression), exhaust backpressure, and induction restriction. These produce large driveability complaints and set no code of their own.

Step 5 — Capture the event. Use the scan tool's record and trigger functions on the suspect parameter during a road test that reproduces the complaint.

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Interrelated System Faults

Task E.9 requires diagnosing driveability and emissions problems resulting from failures of interrelated systems. The PCM does not control engine output alone; it arbitrates requests from several other modules.

Torque management and traction/stability control. The transmission control module requests torque reduction during shifts; the traction and stability systems request reduction during wheel slip or yaw intervention. A faulty wheel speed sensor that intermittently reports slip causes the stability system to command torque reduction on dry pavement — the driver reports "the engine loses power for no reason." The engine is healthy; it is obeying a request. Look for ABS/traction codes and for a torque reduction parameter active in engine data.

Cruise control. Legacy cable and vacuum systems could hold the throttle open mechanically. On electronic throttle vehicles, a cruise control switch shorted in the "resume/accelerate" position produces unexpected acceleration or a refusal to allow the throttle to return to idle. Brake pedal position switch faults disable cruise and can also alter idle and torque strategies.

Security and theft deterrent. An immobilizer that fails to authorize produces a start-and-stall, an extended crank, or a no-start with normal fuel and spark hardware — the pattern covered in section 5.9.

Air conditioning. A/C compressor engagement is an added load the PCM compensates for with idle airflow and timing. A failed A/C pressure switch, a compressor that engages without the PCM's request, or a seized compressor produces stalling on A/C engagement, a low-speed surge, or a belt squeal that presents as an engine complaint. A restricted condenser also raises underhood and intake air temperature, which retards timing.

Charging and cooling load. A cooling fan that never commands on raises coolant and intake air temperature into knock-retard territory; a generator with a seized decoupler adds drag and belt noise.

Non-OEM installed accessories. Remote start systems, alarms, aftermarket audio amplifiers, auxiliary lighting, and trailer wiring are installed by splicing into ignition, starter, ground, and sometimes data circuits. Documented consequences include intermittent no-start, unexplained parasitic drain, ground offsets that shift every sensor signal on a shared return, and network communication faults from an amplifier tapped into a bus wire. Whenever a complaint resists explanation, inspect for aftermarket wiring — the presence of non-factory splices, T-taps, or added modules under the dash reframes the entire diagnosis.

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Strategy Under Test Conditions

ASE writes no-code items as scenarios where the obvious component has already been replaced or already tested good. The correct answer is generally the step that gathers evidence the code list cannot supply: comparing cold-soak sensor values, reading Mode $06 test results, checking a non-monitored mechanical condition, or examining another system's data for a request the engine is dutifully obeying.