MODE $06 MONITOR ID DECODER
Mode $06 (or Service 06 on CAN) returns the raw, non-continuously-monitored test results the ECM uses BEFORE deciding to set a code. A monitor can be failing in the background and you won\'t see a P-code yet — but you\'ll see Mode $06 values drifting past spec. This reference maps the common MID/TID combos for engine performance: catalyst efficiency, O2 sensor response, EVAP leak, EGR flow, and misfire counts.
CATALYST EFFICIENCY (MID $01 – $02)
| MID | TID | Description | Pass threshold | Notes |
|---|---|---|---|---|
| $01 | $80 | B1 catalyst — O2 sensor switch ratio | < 0.500 | Higher = downstream switching faster = cat dying |
| $02 | $80 | B2 catalyst — O2 sensor switch ratio | < 0.500 | Same as above for bank 2 |
| $01 | $81 | B1 catalyst — EWMA (running average) | < 0.500 | If trending up over drive cycles, cat is degrading |
| $21 | $81 | B1 wide-range catalyst monitor | OEM-spec | Used on later vehicles with wide-band sensors |
O2 SENSOR MONITOR (MID $05 – $06)
| MID | TID | Description | Pass threshold | Notes |
|---|---|---|---|---|
| $05 | $01 | B1S1 — Rich-to-Lean response time | < 100 ms | Slower = lazy O2 sensor |
| $05 | $02 | B1S1 — Lean-to-Rich response time | < 100 ms | Slower = lazy O2 sensor |
| $05 | $07 | B1S1 — Min voltage | < 0.150 V | Should swing low cleanly |
| $05 | $08 | B1S1 — Max voltage | > 0.800 V | Should swing high cleanly |
| $06 | $01–08 | B2S1 sensor (same TIDs) | Same spec | Bank 2 upstream |
EVAP LEAK (MID $3D – $3E)
| MID | TID | Description | Pass threshold | Notes |
|---|---|---|---|---|
| $3D | $80 | EVAP 0.040" leak monitor | OEM-spec | Sets P0442 if failing |
| $3E | $80 | EVAP 0.020" leak monitor | OEM-spec | Sets P0456 (small leak) |
| $3C | $80 | EVAP purge flow monitor | OEM-spec | Sets P0441 if purge not flowing |
| $3D | $83 | EVAP vacuum decay rate | slow decay | Fast decay = leak somewhere |
EGR FLOW MONITOR (MID $31 – $32)
| MID | TID | Description | Pass threshold | Notes |
|---|---|---|---|---|
| $31 | $80 | EGR flow — commanded vs actual | OEM-spec | Sets P0401 if low flow |
| $32 | $80 | EGR position feedback | OEM-spec | Sticking valve = position lag |
| $31 | $81 | EGR delta MAP — commanded EGR vs MAP change | > OEM minimum | Low delta = passage clogged |
MISFIRE MONITOR (MID $A1 – $AC)
| MID | TID | Description | Pass threshold | Notes |
|---|---|---|---|---|
| $A1 | $0B | Cyl 1 misfire count, last 200 rev | 0 | Any count = miss event detected |
| $A2 | $0B | Cyl 2 misfire count, last 200 rev | 0 | Same — per cylinder |
| $A1+ | $0C | Misfire count, last 1000 rev (catalyst-damaging) | < threshold | Sets P03XX permanent if exceeded |
| $A1+ | $0D | Misfire count, last 1000 rev (emissions-failing) | < threshold | Sets pending code → MIL on if confirmed |
USING MODE $06 BEFORE THE CODE SETS
When a customer says "P0420 keeps coming back," pull Mode $06 BEFORE clearing. Catalyst MID $01 TID $80 shows you exactly how close to the 0.500 ratio you are. After repairs, watch the value over 50–100 miles — if it\'s creeping up again, you missed an upstream cause. This is the difference between a comeback and a fix.
Same logic for O2 monitors (MID $05–$06) — if response times are trending toward 100ms, the sensor is dying. Replace it BEFORE it sets P0133 and the customer comes back with a Check Engine light.