P1602 — Immobilizer System Communication Error with PCM

STOPIs it safe to drive? Don’t keep driving — risk of real damage or a safety issue.

What P1602 means

This is a manufacturer-specific code — its meaning depends on the vehicle’s make (see below).

What P1602 means on specific makes

Heads up: P1602 is a manufacturer-specific code, so the same number means different things by make.

Mazda STOP

Communication failure between the immobilizer/anti-theft unit and the PCM (some refs also list it as PCM-to-TCM line error — model dependent). On immobilizer-equipped Mazdas this blocks starting until comms/registration are restored.

  1. Immobilizer<->PCM wiring/communication fault
  2. Unregistered key / failed transponder
  3. Weak battery during start
  4. Module replacement needing programming

Check first: Confirm battery health and key registration; resolving usually needs Mazda immobilizer programming. Often grouped with P1260/P1603/P1621.

Source: shadetreespeedshop.com/mazda-check-engine-light

Volvo CAUTION

Volvo's signature defect. On 1999-2002 5-cyl Volvos (S60/S80/V70/XC70/XC90) the Electronic Throttle Module's contacts wear/coke up, causing surging, limp mode, and a constellation of throttle codes. NOTE: Volvo's real codes are native ECM-91xx codes; the exact P1-number a generic scanner shows varies by year, so treat the specific number cautiously.

  1. Worn/dirty Electronic Throttle Module - clean or replace (clean DIY; new module $200-500, reman/repair ~$100-200)
  2. Throttle module internal contacts coked with oil vapor
  3. MAF interaction faults
  4. Wiring/ground

Check first: On the affected Volvos, ETM cleaning often temporarily fixes surging; a reman/upgraded ETM is the durable fix. Volvo extended the ETM warranty historically.

Source: MatthewsVolvoSite ETM article; YouCanic Volvo ETM problems; XeMODeX ETM repair

Related codes

P1260P1603P1621
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